The following summary was extracted from an extensive report on the coalbed methane resource potential of the Richmond Basin prepared by the Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University.
E33 METWE POTENTIAL FROM COAL SEAMS fN THE RICHMOND BASIN OF VIRGINIA
Final Report
March 1981
Frepared by Department of firiing and Minerals ,Engineering Virginia Polytechnic Ins titutE and Stare University Blacksburg, Virginia 24061
under -TRW Subcontract No. HP2176JJ9S ' 'WE C-mtsact NQ D.E-AC21-78MCO8089
for TRW Energy Systems Group 8301 Greensboro Drive McLean, Virginia- 22102
Reprinted with permission of the Department of Mining and Minerals Engineering, Virginia Polytechnic and State University. This report is an evaluation of the coalbed methane resource potential of the Richmond Basin in east-central Virginia. The work was conducted and the report prepared by the Department of Mining and Minerals Engineering, Virginia Polyeechntc Institute and State Uni- versity under TRW/WI & SU Subcontract R12176JJ9S, a subcontract under the U.S. Deparment of Energy (DUE) Methane Recovery from Coalbeds Project administered by the Morgantown Energy Technology Center.
The work develops the geologic framework and the coal and coalbed methane resources in a geologic .basin from which very little recent subsurface exists. This report provides the background OR which future coalbed methane explorztion programs in this basin can be deveioped. The Richmond Coal basin, locared abour: 12 miles west of the cis? of Richmond, Yirghia,vas the site of the first commercial coal mining in the Uniced States. ?fining activity continnu'ed rhese for a period of about 200 years. bespite chis long histcry of mining activity, it is .estimated t'Mt total produc:ion from tne basin only- amounted to about eight million tons of coal. Because very limited exploration work vas done, the full exrEnL of the coal resources of the basin are un- known and resource estimares are tberef ore imprecise , alrhaugt.1 Khey appear to range from 'two to four billion tons. The coal is chiefly medium volatile bi:uminous and in some ereas h2s been altered to coke by igneous intrusions,
From the records, ir appears that the coals contain significanc quantities of methane. Alchough numerous reports of gas, gas explos- ions and disasters are recorded, acegrate measurement ~f the gas con- tent has never been attmpted to establish the potential of this resourct.
The city of Richmond, which currently consumes 13 bFllion cu. ft. per year of natural gas (Lordley, 19791, provides an excellent outlet and market for methane gas which may be produced from the besin.
Geologiczllp, the coal occurs in 2n elongated basin 33 miles from north to south with a width that varies up to 9.5 miles. As scared, exploration has been minimal and the actual proportian of this area of approximztely 170 square miles underlain by coal is not known. However, coal does outcrop ar is known to occur mer 2 distance of about 32.4 miles of a total perimeter of some 70 miles.
0 0 The coal seams dip steeply at 15 - G5 touards the center af the basin. The deepest part of the basin,ar: whi;h coal is known to exist, vas sampled at the Salisbury borehole L+ich intercepted coal at 2320 feet (Jones, 1928) The depth in the center of the basin is not , knom, but could be 3,000 feet or greater. Whether or not the coal is continuous across the basin or ccnrinues around the per: 1 metes CaIXlQK be detemined from present widence.
The ma!. seams mines, ranging from three to five in nuinber, achieved great thicknesses especially where t~oor more of the seams merged. Thicknesses of 70 feet of coal, with some included shale bands, have been recorded.
As a result, assumptions have been made of rhe extent and thick- ness or' the coals, upon which a c021 resource has been es:imared by computer conceuring of available values, which resul~sin B conser- vative estirnare of 2.3 billion tors. The reyrr of Sh2ler and Woodworth (1899) indicates the recoverable coal Teserves ar 1.152 billion tons, which when adjusred LO include the unrecovereble coal gives a resource of 2.1 billion tons. Resource esrimarred by Heinrich (1878) range from 1.987 to 3.887 billion tons. In this report, how- ever, a more conservative estimzte of two billion ton5 has been used for the calculacions of the methane conrenr.
These resources were concourgd for depth and thickness of caal, and an average quality of the coal wa5 calculated. The results,based on the available data pertaining ch-ier‘ly t5 the perimeter vhich was mined, are of varying credibility because of the quality dates and origin of some of the reports. For this rezson, the gas content has been estimated using the method of W (19771, and the result has been evzluared for csedkbility with the reports of methane g+s and found to have no conflict.
From this study it would appear that the potentialresource methane may range from 700 billien cu. ft. tolGOO billion cu. ft. or enough to provide many years supply based upon the current ‘tat21 usage rate of Richmond.
Therefore, the drflling of 2 nuaber of exploratory holes in the bash may have merit EO establish the actllzl exfenr of the coal beds, the merhane content of these Sed5,and the characteristics of desorp- tion of the methane gas from the beds. GUIDEBOOK TO THE GEOLOGY OF THE RICHMOND AND TAYLORSVILLE BASIN, EAST-CENTRAL VIRGINIA
By Bruce K. Goodwin1, Robert E. Weems2, Gerald P. Wilkes3, Albert J. Froelich2, and Joseph P. Smoot2
Eastern Section AAPG Meeting Fieldtrip Number 4
10 November 1985
1 College of William and Mary, Williamsburgh, VA 23185 2 United States Geological Survey, Reston, VA 22092 3 Virginia Division of Mineral Resources, Charlottesville, VA 22903
The following guidebook, prepared by Dr. Bruce K. Goodwin and others in 1985 for the Eastern Section Meeting of AAPG, was available for many years from the Virginia Division of Mineral Resources in Charlottesville, VA. It is reproduced herein, with permission of the Division, to replace the now out-of- print version of the guidebook. Content s
Introduction and regional setting ...... 1 Areal extent and geologic setting ...... 3 Previous work...... 9 Stratigraphy ...... 9 Stratigraphy of the Richmond basin ...... 10 A brief history of coal mining in the Richmond basin. .. 12 Stratigraphy of the Taylorsville basin ...... 14 References cited...... 16 Road log ...... 19 Taylor svi 1 le bas i n Stop 1 -- Doswell Formation, Stagg Creek Member and Falling Creek Member...... 19 stop 2 -- DoswelI Formation, Newfound Member..,.,..... 28 Stop 3 -- Doswell. Formation, Newfound Member...... 30 Stop 4 -- Doswell Formation, Newfound Member ...... 32 Ri chmond basin Stop 5 -- Petersburg Granite and Tuckahoe Group, lower barren beds...... 35 Stop 6A -- Chesterfield Group, Vinfta Beds...... ,.... 40 Stop 6B -- Chesterfield Group, Vinita Beds...... 43 Stop 7 -- Tuckahoe Group, productive coal measures.... 45 Stop 8 -- Chesterfield Group, Otterdale Sandstone ..... 54
F i gur es
1 -- Exposed basins of the Newark Supergroup in eastern North America...... 2 2 -- Regional geologic setting of the Richmond and TayPorsville basiRs ...... 4 3 -- Generalized geologic nap of the Richmond basin and outlying basins.., ...... 5 4 -- Geologic map of the exposed portions of the Taylorsville basin ...... 6 Figures [continued)
5 -- Stratigraphic columns of the Richmond and TaylorsviIle basins...... *...... - 8 6 -- Generalized map of the geology surrounding the Richmond and Taylorsville basins, also showing the status af 7.5-minute geoIogic quadrangle mapping in both basins...... ,..,.... IS 7 -- Map showing detailed location of Stops 1 and 2 in the Hanover Academy ?.5-rninute quadrangie ...... 20 8 -- Columnar section of Outcrop A at Stop l...... 22 9 -- Columnar section of Outcrop B at Stop I...... 27 10 -- Diagramnatic portrayal of Outcrop C at Stop 2...... 29 11 -- Map showing detaiIed location of Stops 3 and 4 in the Ashland 7.5-minute quadrangle ...... 31 12 -- Geologic map of Stop 5 and vicinity ...... 36 13 -- Generalized geologic section of the Triassic rocks exposed in the Boscobel Quarry...... 38 24 -- Geologic nap of Stop 6 and vicinity...... 41 15 -- Geologic map of Stop 7 and vicinity ...... 46 16 -- Sketch map of the exposed coal beds and basin margins near Midlothian, and hypothetical cross sections across the basin margins ...... 47 17 -- Geologic map of Stop 8 and vicinity ...... 55 GUIDEBOOK TO THE GEOLOGY OF THE RICHMOND AND TAYLORSVILLE BASINS, EAST-CENTRAL, VIRGINIA
After the Appalachian orogeny, the long period from Permian through Early Triassic time was dominated by erosion and nondeposition in eastern North America. This interval was succeeded by an early Mesozoic rift regime, apparently controlled by extensiond forces, that eventually would open up the AtIantic Ocean basin. One of the earliest signs of this profound change in tectonic regime was the commencement of deposition in the basins of the Newark Supergroup (fig, 1). The basal strata of the Fundy basin we Ladinian (Middle Triassic) in age and are the oldest known Newark beds (Froelich and Olsen, 19841, so this northernmost basin may have been the first to develop. Shortly after, probably in the middle Carnian, the Richmond and TayIorsville basins also began to subside and accumulate sediments. By late middle to late Carnian time, the other major rift-related basins had developed along inherited tectonic trends.
Diverse pollen and spore floras and the fluvid to lacustrine sedimentary fill of the various Newark basins in middle Carnian time suggest that these sediments were deposited under at Ieast seasonally wet conditions. A trend toward increasing regiond aridity is suggested by the alternating chemically precipitated and detritalIy accumulated sediments of the upper Carnian lake sequences in the Newark basin (Van Houten, 1964) and by upper Carnian floras in the DanvilleJDan Rives basin (Cornet, 1977; Olsen and others, 1978; Robbins, 1982). By Norian time, the basins apparently became semiarid to intermittently arid, as indicated by reduced diversity among pollen and spores (Cornet, 1977). Dry conditions continued to prevail into earIiest Jurassic time, but later during the Early Jurassic, conditions became more moist and equable, as indicated by the increased abundance of organic matter-rich lake deposits and stream deposits and by an increasing diversity of spores and pollen (Cornet, 1977). A second interval of aridity may have occurred near the end of Newark time, for pseudomorphs of evaporite minerals are found in the highest beds of the Newark and Hartford basins (J. P. Smoot, personal observation).
Depositional interludes in the Early Jurassic were punctuated by at least three major episodes of basaltic extrusion, which produced well-preserved flow sheets in all basins from the Culpeper north- Numerous diabase dikes, which cut the Triassic strata of the older southern basins, allso may be associated with this volcanic activity. Subsequent EXPLANATION
1. Wadesboro (N.C. - S.C.) 2. Sanford (N.C.) 3. Durham IN.C.1 4. hieCounty tN.C.1 5. Dan River and Qanville 4N.C.- Yam) 6, Scottsburg (Va.) 7. Basins north of Scottsburg (Va. 1 8. Farmville (Va.1 9. Richmond (Va.) 10. Taylorsvilk (Va.1 11, Scottsville (Va.) 12, Barboursville IVa.1 13. Culpeper (Va. - Md.1 14. Gettysburg EMd. - Pa.) f5. Newark IN.J.- Pa. - N.Y.1 16. Pomperarrg (Conn.1 17. Hartford (Conn. - Mass.) 18. Deerfieid (Mass.) 19. Fundy or Minas (Nova Scotia - Canada) 20. Chedabucto {Nova Scotia -Canada)
0 100 200 3OQ MllES I+- I+- -.,. _. -1 0 100 ZOO 300 400 KILOMETERS
Figure 1. - Exposed basins of the Newark Supergroup in eastern North America (from Froelich and Olsen, 1984). tectonism, which accompanied the opening of the Atlantic to the east, produced the tilted half-grabens that now characterize the Newark Supergroup of eastern North America.
This trip will be mainly concerned with sediments that accumulated during the early, relatively wet stage of this sequence, and with the subsequent diagenetic and tectonic changes that transformed them into the rocks that me ROW present in the preserved remnants of the Richmond and Taylorsville basins.
The Richmond and Taylorsville basins in east centrd Virginia (fig. 2) are north- northeast-trending fault-controlled troughs that contain reaonalIy northwest-dipping Upper Triassic sedimentary rocks of the Newark Supergroup. The two basins are part of a series of half-grabens, all of early Mesozoic age, that are located east of the folded Appalachians (fig. I). Locally, strata are cut and metamorphosed by narrow dikes of tholeiitic diabase which are probably of Early Jurassic age. Although all of the Newark basins possibly had similar depositional and early structural histories, they ROW are found in two very different geographic settings. The more westerly (exposed) basins are in sub- parallel belts in the Piedmont and equivdent terranes in New England and Maritime Canada from South Carolina to Nova Scotia (fig. I), whereas mare easterly basins are buried beneath Cretaceous to Quaternary sediments of the Atlantic Coastal Plain and Continental Shelf from Florida to Nova Scotia.
The Richmond basin (fig. 3) is located 12 miles (19 km) west of the Atlantic Coastal Plain and the City of Richmond; its approximate geographic center is just southeast of Hallsboro in Chesterfield County. The basin is 33 miles (53 km) long, with 8 maximum width of about 9.5 miles (15 km), and is exposed over an area of approximately n 190 square miles (500 km'). Five smaU outlying basins in proximity to the northeastern end of the Richmond basin were probably once continuous with the Richmond basin but have since been partially or completely isdated by erosion.
The Taylorsville basin (fig. 4) is located 16 miles (26 km) north of the City of Richmond and 7 miles (11 kin) northeast of the Richmond basin. The exposed part of the Taylorsville basin is 12 miles (19 km) long and a maximum of 7 miles (11 km) wide, with an areal extent of about 5a square miles (130 ktnz). The total area of this basin is Imger, probably much larger, because the northeast edge of the exposed basin is overlapped by sediments of the Atlantic Coastal Plain. Subsurface control beneath the Coastal Plain is presently insufficient to delineate accurately the TayIorsvilJe basin beneath it, but Figure 2. - Regional geologic setting of the Richmond and Taybrsville basins (modified from Johnson and others, 1985). N
Figure 3; - Generalized geologic map of the Richmond basin and outlyiig basins (modified from Shaler and Woodworth, 1899; Roberts, 1923; Goodwin, 1973; and Goodwin and Farrell, 1979). Locations af field trip stops 5 through 8 are indicated. Figure 4. - Geologic map of the exposed portions of the Taylorsville basin (modified from Weems, 1980b). Locations of field trip stops 1 through 4 are indicated. geophysical and sparse drill-hole data suggest that it codd extend northeast into southern Maryland (Robbins and others, in press).
The Richmond basin is completely enclosed by older igneous and metamorphic rocks of the Piedmont Province (fig. 2). The exposed parts of the Taylorsville basin are simiIarIy enclosed, as probably are the buried parts of the basin. Beds in both the Richmond and Taylorsville basins show a regional northwest tilt and both basins are bounded OR their northwest sides by a common tectonic feature, the Hylas zone. This fault zone, composed mainly of catadastic rocks, has been active in both late Paleozoic and early Mesozoic time (Bobyarchick and Glover, 19791, and in early to middle Tertiary time as well (Mixon and Powars, 1984). Most clasts in Triassic conglomerates along the northwest basin borders seem to represent fragments of the Hylas zone cataclastic rocks. Angular clasts are found in some of the Triassic conglomerates near the top of the sedimentary sequence in the Taylorsville basin (fig. 5), and along the western margin of the Richmond basin north of the James River. These may represent talus-slope deposits locally preserved at the base of the fault scarps from which they were derived. These observations suggest that the western margins of the basins, at least during the later stages of their development, were rooted in the intermittently active Hylas fault zone. Much of the sediment in both basins WBS derived by weathering and erosion of the gneiss, amphibolite, granodiorite, and granite of Proterozoic to possibly early Paleozoic age that are found immediately northwest of that zone.
The Petersburg Granite complex flanks both basins on the other exposed margins, except at the southern end of the Richmond basin where rnetasediments are exposed (fig. 3). The sedimentary deposits of the Richmond and TaylorsviUe basins unconformably overlie the Petersburg Granite, and the conglomerates exposed along the eastern basin margins contain numerous clasts of Petersburg Granite. This granite has been dated as 330+,8 million years in age (Wright and others, 1975). The presence of two micas (biotite and muscovite) in at least some of the Petersburg facies indicates that the granite may have formed at a depth of at least 5 miles below the surface of the earth. Its presence directly below strata of the Richmond and Taylorsville basins therefore suggests that erosion may have stripped at least 5 miles of material from the complex of Petersburg batholiths during the 100 million years before the Late Triassic. Although small-scale faults are locally found along the eastern margins of the basins, the strata generany dip gently toward the west. Their structural and stratigraphic configuration, BS well as the small erosional outliers of the Richmond basin, suggest that in the past both basins extended farther to the east. I UPPER TRIASSIC 1 I MIDDLE CARNIAN :$UPPER CARNIAN~
72-b
a -3 m ‘5 w4 5. 3 c (0 Previous work
One of the earliest worh on the Triassic rocks of the Richmond basin was by Sir Charles LyeU (1847). Other early publications on the coals, fossils, stratigraphy, and Structure of this basin me by Heinrich (1878), Fontaine (18831, Ashburner (1888), Shaler and Woodworth (1899), Woodworth (1902), and Roberts (19233. The first gmIogiic map of the basin was produced by Shaler and Woodworth (18991, reproduced by Roberts (1928) with no change, and provided the outIine of the basin shown on the Geologic Map of Virghia (Milici and others, 1963). More recent studies of the Richmond basin and surrounding areas have been made by Goodwin (1970, 1980, 19813, and Goodwin and Farrell (1979). Studies of the paleontology and the age and correlation of the sedimentary rocks as revealed by spores, pollens, vertebrates, and other fossils have been made by Redfidd (18411, Fontaine (18831, Cornet, Traverse, and McDonald [1913), Cornet (19771, Schaeffer and McDonald (19781, Olsen, McCune, and Thompson (19821, and Olsen 11984). Cornet (1977)wiped all of the Triassic strata in the Richmond and TaylorsvilIe basins to his Chatham-Richmand-TayIorsville palynofloral zone of middle Carnian age (Late Triassic), and Olsen, McCune, and Thompson (1982) include these beds in the Dictyopyge fish zone. The approximate correlation of strata between the Richmond and Taylorsville basins is indicated in figure 5. The sedimentary rocks in the Taylorsville basin were first studied and recognized as being Triassic or Jurassic in age by Rogers in the 18313's and 1840's (these reports are reprinted in Rogers,1884). Heinrich (1878) and Russell (1892)briefly discussed the strata in this basin, but after their time the sediments in the Taylorsville area were considered to be either mostly part of the Cretaceous Patuxent Formation (Clark md Miller, 1912) or to represent only a small outlying part of the Richmond basin (Roberts, 1928; Krynine, 1950). No detailed mapping was done in the Taylorsville basin until the work of Weems (1980b, 1981, 1985), who aIso introduced the present stratigraphic nomenclature for this basin (figs. 4, 5). Aspects of the paleontology of this basin have been studied or discussed by Fontaine (1883), Knowlton (1899), Cornet, Traverse, and McDonald (19131, Cornet (19771, Schaeffer and McDonald (19781, Weems (1980a1, Olsen, McCune and Thompson (1982), and Olscn (1984).
Stratigraphy
The Triassic lithologies present in the two basins me similar, being dominantly sandstones with subordinate amounts of con$om erate, siltstone, shale, coal, and limestone. The sequence of these sediments in the two basins is somewhat different and the timing of sedimentation is not entirely synchronous (fig. 51, so the stratigraphic columns of the two basins have been named separately and are discmsed separately below .
Stratigraphy of the Richmond basin:
Shaler and Woodworth (1899) divided the Triassic rocks in this basin into two groups: a lower, Tuckahoe Group and an upper, Chesterfield Group. Olsen and others (1978) have retained these two group names to refer specifically to the rocks of the Newark Supergroup in the Richmond basin. These two groups were subdivided by Shder and Woodworth, who divided them, from bottom to top, into the Boscabel boulder beds, the lower barren beds, and the productive cod measures within the Tuckahoe Group, and the Vinita Beds and the Otterdale Sandstone within the Chesterfield Group. Variations on these names are still in common usage, but terminology for the Richmond basin has not stabilized. The major cod-mining districts in the Richmond basin are also indicated on figure 3. Since 1980, 15 exploratory test holes for oil and gas have been drilled to total depths ranging from 1,195 ft (362 m) to 7,443 ft 12,255 m). Most had gas and/or oil shows and only 4 penetrated the crystalfine basement rocks.
The Boscabel boulder beds, as described by Shaler and Woodworth (18991, consist of lmge angular blocks of cataelastic rocks, as much as 2 by 3 feet (0.7 by 1 m) with some larger than 5 feet (1.7 m) in diameter, in a matrix of Triassic sandstone. They were referred to as a boulder breccia by Goodwin and Farrell (1979)and are found in a narrow outcrop belt (Fig* 3) dong the faulted western margin of the basin west of Boscobel (which is R variant spelling of Bascabel) and north of the James River. They have not been recognized elsewhere dong the basin margin, possibIy because of the paucity of exposum within the basin. The angular nature, random orientation, and localized distribution of the Boscabel boulder beds suggest that they were deposited BS talus piles at the base of a fadt scarp along the western margin of the basin. It is probable that they are not a basal Triassic unit across the entire basin as envisioned by Shder and Woodworth, but rather that they were deposited Iocally along the faulted northwest margin and proceed to Fade eastward into finer grained sediments {Goodwin and Farrell, 1979). CongIomerates aIsa are found ZocaIly along the eastern and southern margins of the Richmond basin (fig. 3).
The lower barren beds are so named because they lack coal and underlie the coal measures. The best section through the lower barren beds was presented by Heinrich (18781, who compiled a stratigaphie column from mine workhgs and drill hoIes in the Midlothim area. He showed a thickness of 571 Et (173 rn) for the lower barren beds including 36 ft (11 rn) of basal conglomerate, but thicknesses are usually less than 400 ft (130 m). Heinrich described the rocks above the basal conglomerate and below the coal. measures as being about 72 percent sandstone and 28 percent shale. The sandstones we light to dark gray, commonly carbonaceous and arkosic, whereas the shales are black or brownish bIack, and commonly rich in organic matter. Heinrich mentioned that about half of the shales in the upper part of the lower barren beds are calcareous, containing streaks and concretions of calcium carbonate. He also stated that the shales contain plant fossils, fish scales, and some reptilian teeth.
The productive cod measures constitute only a minor part of the stratigraphic column above the lower barren beds. Other coal beds locally occur at higher levels within the Vinita Beds and the Otterdale Sandstone. The base of the lowest minable coal marks the boundary between the lower barren beds and the productive coal measures, and the latter unit ranges from 0 to 400 ft (122 m) in thickness. Estimates of composite coal thickness within the productive coal measures have been: 32 ft (9.6 rn) of cod in 90 ft (27 m) of section in the Midlothian area (Heinrich, 187s); 43 ft (13 rn) of coal in 180 ft (54 rn) of section at Winterpock (Fontaine, 1883); and 25 ft (7.5 rn) of coal within 112 ft (34 rn) of section at Gayton (Davis and Evans, 1933). The coal is usually 8 goo+quaIity bituminous coal which k rich in methane. Black shales and fine- to medium-grained carbonaceous sandstones are present with the cod. Natural coke has been formed Iocally where EarIy Jurassic(?) dikes intrude CO~Iseams.
The section described by Heinrich (1878) was composed of approximately 35 percent sandstone, 36 percent cod, and 26 percent shde (much of which is probably siltstone). The sandstone is gray and in places arkosic. It is generally poorly sorted, containing rounded grains that are bonded by silica or caIcite cement. Cod bands, rafted coal, and plant debris have been notd in sections by Heinrich (1878) and Shder and Woodworth (1899). The sandstone occurs as massive bodies or is interbedded with shale, siltstone, or coal. Shale occupies only a small part of the section. Locally, fossiliferous carbonaceous shale is found just above or beIow some coal seams which grade into other lithologies within a few feet. Rooted underclays underlie some cod seams.
The Vinita Beds overlie the productive cod' measures and consist dominantly of fine to coarsegrained, gray sandstone (in part crossbedded), gray siltstone, and dark- gray to Hack shale. Some conglomerates, containing well-rounded pebbles of quartz, granite, and gneiss are present within the VInEta Beds, at the base of channels cut into underlying beds. ShaIer and Woodworth (1399) considered the Vinita Beds to be about 2,000 ft (600 rn) thick. However, recent drilling in the Richmond basin has shown that the Vinita Beds may be as much as 6,000 ft (1,300 rn) thick. They were probably deposited in lacustrine, fluvial, and deltaic environrn ents, with lacustrine sequences dominating the center of the basin and fluviedeltaic sediments extending into the basin from the margins. The fI uvbdeltaic deposits are apparently depcsitionally localized along the fauIted western-margin of the basin. The everlying Otterdale Sandstone forms a large, lobate mass extending from the western border eastward toward the center of the basin (fig. 3). The predominant lithology is coacsegrained, arkosic sandstone that is commonly cross-bedded and contains well-rounded quartz grandes and pebbles. Prominent channel scours are filled by coarsegrained, crossbedded, pebbly sandstone with basal cobble zones. Conglomeratic beds containing well-rounded pebbles and cobbles of quartz, gneiss, and granite, also are common. Finegrained sandstones are locally interbedded with siItstones and shales. The conspicuous channels and their fill of coacsegrained, cross- bedded sandstones and conglomerates suggest that these sediments were deposited in a fluvial environment. A braided stream network that aIternateIy cut and filled the surface of the area probably produced the numerous channels, and intermittent overbank deposits of silt produced the interfingecing micaceous siltstones.
Shaler and Woodworth (1899) determined that the Otterdde Sandstone was more than 500 Et (150 m) thick, but they were unabIe to estabIish Its full thickness. A well, drilled 0.5 mile from the basin's western border in 1978, penetrated 1,514 ft (454 rnl of coarse sandstone and conglomerate comparable to the Otterdale before terminating in a diabase (Goodwin and FarreU, 1979). This suggests that the Otterdale Sandstone might continue down to basement near the northwestern edge of the basin, and that the lower strata of the unit may have been deposited in facies contact with lacustrine Vinita Beds, which were accumulating contemporaneousIy in the interior of the basin. The Otterdale thins to the northeast, becomes less conghmeratic, and there overlies the Vinita Beds. Locally, the OtterdaIe Sandstone and Vinita Beds appear to interfinger. This nay have been caused by intermittant pulses of basin deepening, which would have affected the volume of coarse clastic sediments which could have been carried into the basin and deposited at the distal eastern end of the OtterdaIe sequence.
A brief history of coal mining in the Richmond basin: The first cod mined in the United States came €rem the Richmond basin (Robbins and others, in press). Although coal was reported from this area as early as 1701, active mining did not commence until around 1750 (NichoUs, 1904) and only in 1795 did the annud production from the basin exceed 2,000 tons. Since then, three major mining centers developed on the eastern margin of the basin: (1) the Gayton District (Carbon Hill District) north of the James River; (2) the Midlothian District south of the James River; and /3) the Winterpock District (Clover Hill District) new the southern end of the basin. TWQmining centers developed on the western margin of the basin: (4) the Manakin District north of the James River; and (5) the Huguenot Springs District south of the James River. Mining was nearly continuous along the eastern margin o€ the Richmond basin between the Gayton and Midothian Districts, and on the western margin between the Manakin and Huguenot Springs meas. On the eastern margin, where Triassic roc@ generally unconforrnably overlie basement, many mines began BS open pits on the coal exposures and were later extended as indines down the dip of the coals. Some shafts were driven down to the coals west of the exposures and inclines were then extended into the coals at the base of the shafts. The westward-dipping Triassic basement contact and the overIying cod measures on the basin's eastern ma& had many flexures (or "troubles" in the terrninobgy of the miners). These caused the coal to thicken and thin rapidly, and even to pinch out locally. Adjacent to the faulted western margin of the basin, shaft mines were the rule. Although the coal in the Richmond basin is a high volatile bituminous coal of good quality, extracting it in this area has always been dangerous and frustrating. Roof control was difficult, violent methane expIosions were common, and structural instabilities were difficult to predict.
At least 2,842,645 tons of coal were produced between 1780 and 1840 (Brown and others, 1952, Table 5). Most of this coal was used locally for domestic purposes, but some was shipped from Richmond to Philadelphia and New York. Mining activity was at its peak and flourished around Winterpock, Midothim, and Gayton from 1842 to 1880. North of the James River, most of the mines around Manakin and Gayton finally closed around 1880 except for one deep, 2400-foot-long incline at Gayton which was open until 1912. Peak annual production was in 1835 with 201,000 tons of coal. This dropped to only 50,000 tons by 1923 (Roberts, 1928), when four mines were operating and only one WBS shipping mal. Since 5930, coal production has been negligible. Between 1935 and 1937 three small-scale CO~mining ventures were reported in She newspapers: (1) a dragline operation in the Black Heath area; (2) a shallow incline near Winterpock; and (3) a shaft in the Huguenot Springs area.
It is unknown how much coal remains beneath the surface of the Richmond basin. Most of the former mining was confined to the basin's margins, and until the recent oil exploratory driIling activity nothing was known about the thickness, depth, or continuity of the coal measures across the basin. The old estimate by Shaler and Woodworth (1899) of 1,152,000,000 tons of coal remaining in the Richmond basin is'probably reasonable, but the depth to the coal at the center of the bash is now known to be double their original estimate.
Stratigraphy of the TayIorsville basin:
The exposed parts of the Taylorsville basin we within the Ashland, Hanover Academy, and Ruther Glen quadrangles (fig. 6). The entire stratigmphic column of the Taylorsville basin (fig. 5) has been named the Doswell Formation, and it has been subdivided into three members (Weems, 11980b). The basal Stagg Creek Member consists of about 600 ft (180 rn) of sandstone and conglomerate. Above the Stagg Creek are about 1200 ft (360 m) of paludal to lacustrine, interbedded flaggy sandstone, siltstone, shale, coal, and limestone which constitute the Falling Creek Member. Fossils of clams, branchiopods, fish, and reptiles are common locaUy. The coals probably occur in at least two horizons, one near the middle of the member and one near the tup. Although they were mined on a small scale, there is no evidence that either of the coal-bearing intervals had much economic potential. The Falling Cseek may be a potential hydrocarbon source bed, and for this reason it has received attention from oil exploration geologists in recent years. To date only four shallow coreholes, 145-225 ft (43-67 rn) deep, have been drilled into the exposed basin, one of which was used in constructing one of the Cross section lines for the Ashland quadrangle (Weems, 1985). Deeper holes drilled farther northeast through the Coastal Plain have penetrated similar strata which probably belong to this basin.
Above the Falling Creek, a thick (as much as 1,000 ft or 300 m) sequence of massive to crossbedded, minerdogically immature sandstone and conglomerate constitute the sandstcrneJconglomerate facies of the Newfound Member. This sequence is more conglomeratic toward the northwest border fadt. The clasts are generally rounded to subrounded. The sandstones found farther into the basin typically consist of lenticular, 13-16 ft (4-5 m)-thick, fining-upward sequences with trough crossbedding; these are interpreted as braided river deposits. No exposed finegrained beds aw correIative to these river deposits within the basin, which suggests the possibiIity that 1) the paleodrainage was open to allow the finest fractions of sediment and dissolved materials to be transported out of the basin, or 2) that the depoeenter of the basin is now buried beneath the Coastal PIain. Above this sandstonelconglomerate interval is a thick (as much as 2,000 ft or 600 rn) sequence of siltstorm and massive sandstones. These beds 37" 45'
37" 30'
37' 15'
Figure 6. - Generalized map of the geology surrounding the Richmond and Taylorsville basins, also showing the status of 7.5-minute geologic quadrangle mapping in both basins. are extensively bioturbated, probably by plant roots, so that most of their primary depositional structures have been obliterated; therefore these deposits are interpreted to have accumulated OR vegetated flood plains or under submerged aquatic vegetation. Tectonically, they probably represent a period of slow deposition and uplift, analogous to the interval in which the Falling Creek Member was deposited, but without the development of a Iake within the exposed part' of the basin. In outcrop, these beds are poorly consolidated and crumbly. Local drilIers have reported that below a depth of about 200 ft (60 m) this unit produces only salt water, and minor amounts of gypsum have been reported in the least weathered sediments of the basin (Weerns, t980a). -4 loss of gypsum and possibly carbonate cement would explain the anomalously soft, crumbly nature of these sediments in outcrop. Locally, at the top of the Newfound Member, angular cobble and boulder conglomerates dong the northwest border of the basin suggest the former presence of talus slopes at the base of an exposed fault scarp.
References cited
Ashburner, C. A., 1888, Coal: U. S. Geological Survey Mineral Resources, 1887, p. 168- 382. Bobyarehick, A. R., and Glover, Lynn, ILI, 1979, Deformation and metamorphism in the Hylas zone and adjacent parts of the eastern Piedmont in Virginia: Geological Society of America Bulletin, Part I, v. 90, p. 739-752. Brown, Andrew, and others, 1952, Coal resources of Virginia: U. S. Geological Survey Circular 171, 57 p. Clark, W. B., and Miller, €3. L. , 1912, The physiography and geology of the Coastal Plain province of Virginia, with chapters on the Lower Cretaceous, by Edward W. Berry, and the economic geology, by Thomas Leonard Watson: Virginia Geological Survey, Bulletin 4, p. 13-222. Cornet, Bruce, 1977, The palynostratigraphy and age of the Newark Supergroup: University Park, Pennsylvania State University, unpublished PhD dissertation, 506 P- Cornet, Bruce, 1985, Structural styles and tectonic implications of Richrnond- TaylorsviUe rift system, eastern Virginia (abstracth American Association of Petroleum Geologists Bulletin, v. 69, p, 1434-1435. Cornet, Bruce, Traverse, Alfred, and McDonald, N. G., 1973, Fossil spores, pollen, and fishes from Connecticut indicate Jurassic age for part of the Newark Group: Science IAAAS), v. 182, p. 1243-1247. Davis, I. F., and Evans, L. S., 1938, The Richmond coal basin-A compilation in three parts: Library of Congress, Typewritten and bound. Fontaine, W. M., 1883, Contributions to the knowledge of the older Mesozoic flora of Virginia: U. S. GsoIogical Survey Monogmph VI, p. 1-144. FroeGch, A. J., and Olsen, P. E., 1984, Newark Supergroup, a revision of the Newark Group in eastern North America: U. S. Geological Survey Bulletin 1537-A, p. A%- A58. Gooch, E. W., Wood, R. S., and Parrott, W. T., 1960, Sources of aggregate used in Virginia highway construction: Virginia Division of Mineral Resources, Minerd Resources Report 1, p, 31. Goodwin, B. K,, 1970, Geology of the Hylas and Midlothian quadrangles, Virginia (1: 24,000): Virginia Division. of Minerd Resources Report of Investigations 23, p- 1-51. Goodwin, E, K., 1930, Geology of the Bon Air quadrangle, Virginia (1:24,OQOk Virginia Division of Mineral Resources Publication 18 (GM 127). Goodwin, 5. K., 1981, Geology of the Glen Allen quadrangle, Virginia (1:24,000$: Virginia Division of Mineral Resources Publication 31 (GM 127A). Goodwin, B. K., and FarreU, K. M., 1979, Geology of the Richmond basin, in Geology and coal resources of the Richmond Triassic basin: Virginia Division of Mineral Resources Open-File Report, p. 1-43. Heinrich, 0.J., 1878, The Mesozoic Formation in Virginia: Transactions of the American Institute of Mining Engineers, v. 6, p. 227-274. Johnson, S. S., Wilkes, G. P., and Gwin, M.R., 1985, Simple Bouguer Gravity anomaly map of the Richmond and TaylorsviUe basins and vicinity, irirginia: Virginia Division of Mineral Resources Publication 53 (1:125,OOO). Johnson, W. R., 1846, Bituminous coking coals from the eastern coalfields of Virdnia, in the neighborhood of Richmond: Navy Department, 28th Congress, Senate Numbet 336, p. 308-451. Knowlton, F. H., 1899, Report on some fmssil wood from the Richmond Basin, Virginia: U. S. Geological Survey Annual Report 19, part 2, p. 596-519. Krynine, P. D., 1950, Petrology, stratigraphy, and origin of the Triassic sedimentary rocks of Connecticut: Connecticut Geologzcal and Natural History Survcy Bullctin 73, 239 p. Lyell, Charles, 1847, On the structure and probable age of the coal field of the James River, near RichmQnd, Virginia: Geological Society of London Quarterly Journal, V. 3, p- 261-281). Milici, R. C., Spiker, C. T., Jr., and Wilson, J. M., 1963, Geologic Map of Virginia: Virginia Division of Mineral Resources (scale I: 500,0001. Mixon, R, B., and Powms, D. S.? 1984, Folds and faults in the inner Coastal Plain of Virginia and Maryland; their effect on distribution and thickness of Tertiary rock units and local geomorphic history, in Frederiksen, N. O., and Krafft, Kathleen (eds.), Cretaceous and Tertiary Stratigraphy, Paleontology, and Structure, Southwestern Maryland and Northeastern Virginia: American Association of Stratigraphic Palynologists Field Trip Volume and Guidebook, p. 112-122. NichoUs, W. J,, 1904, The story of American coals: Philadelphia, Pennsylvania, J. B. Lippincott Company, 405 p. Oken, P. E., 1984, Comparative paleolimndogy of the Newark Supergroup: a study of ecosystem evolution: New Haven, Connecticut, Yale University, unpublished PhD dissertation, 726 p. Olsen, P. E., Rernington, C. L., Comet, Bruce, and Thornson, K. S., 1978, Cyclic change in Late Triassic lacustrine communities: Science, vol. 201, no. 4357, p. 729-733. Olsen, P. E., McCune, A. R., and Thompson, K. S., 1982, Correlation of the early Mesozoic Newark Supergroup by vertebrates, principdly fishes: American Journal of Science, V. 282, p. 1-44. Redfield, W. C., 1841, Short notices of American fossil fishes: American JournaI of Science and Art, volume 41, p- 24-28. Robbins, E. I., 1982, Fossil Lake Danville: The paleoecology of a Late Triassic ecosystem on the North Carolina-Virginia border: University Park, Pennsylvania State University, unpublished PhD dissertation, 400 p. Robblns, E. I,, Wilkes, G. P., and Textoris, D. A., in press, Coals of the Newark rift system in Manspeizer, Warren, ed., TriassieJurassic rifts and the opening of the AtEantiF Americm AssQc~~~~o~of Petroleum Geologists Memoir, 35 p. Roberts, 3. K., 1928, The geoIogy of the Virginia Triassic: Virginia Geological Survey Bulletin 29, 205 p. Rogers, W. B., 1884, The geology of the Virginis: New Pork, D. Appleton and Company, 832 p. Russell, I. C., 1892, The Newark System: U. S. Geological Survey Bulletin 85, p. 1-344. Schaeffer, Bob$, and McDonald, N. G., 1978, Redfieldiid fishes from the Triassic-Liassic Newark Supergroup of eastern North America: Bulletin of the American Museurn of Natural History, v. 159, wticle 4, p. 131-173. Shaler, N. S. , and Woodworth, J. B. , 1899, Geology of the Richmond basin, Virginia: U. S. Geological Survey AnnuaI Report 19, 1897-1898, part 2, p. 385-515. Van Houten, F. B,, 1964, Cyclic lacustrine sedimentation, Upper Triassic Lockatong Formation, central New Jersey and adjacent Pennsylvania: Kansas Geologicd Survey Bulletin 189, p. 497-531. Weems, R. E., 1980a, An unusual newly discovered archosaur from the Upper Triassic of Virginia, US.A.: Transactions of the American Philosophical Society, w. 70, part 7, p* 1-53, Weems, R. E., 1980b, Geology of the Taylorsville basin, Hanover County, Virginia, Contributions to Virginia Geology - Ilk Virginia Division of Mineral &sources Publidation 21, p. 23-38. Weems, R. E., 1981, Geology of the Hanover Academy quadrangk, Virginia (1:24,000): Virginia Division of Minerd Resources Publication 30 (GM 150D). Weems, R. E., in press, Geology of the Ashland quadrangle, Virginia (1:24,000): Virginia Division of Mineral Resources Publication. Woodworth, J. B., 1902, The Atlantic Coast Triassic coal field: United States Geological Survey Annual Report 22, 1900-1901, part 3, p. 25-53. Wooldridge, A. S., 1842, Geologiccal and statistical notice of the coal mines in the vicinity of Richmond: American Journal of Science, v. 43, p. 1-14. Wright, J. E., Sinha, A., and Glover, Lynn, m, 1975, Age of zircons from the Petersburg Granite, Virginia: with comments on belts of plutons in the Piedmont: American Journal of Science, v. 275, no. 7, p. 848-856. ROAD LUG
Begin Trip-Leave Williamsburg via chartered bus. Take Interstate 64 west to Interstate 295, bear right (northeast) to Interstate 95. Bear right (north) ta Va. 54: BanoverlAshland" exit at milepoint 92.1 on Interstate 95. Take Ashland exit (Va. Route 54 West). This entire stretch of travel is through the CoastaI Plain Province. Detailed road log corn rnences.
mileage -total incrern ent 00.0 0.0 Cross over Interstate 95 going west QTI Ya. Route 54. 00.6 0.6 At U.S. Route I continue west through traffic light on Va. Route 54. The boundary between the Petersburg Granite and the strata of the TayIorsvikle basin IStagg Creek Member) is in this vicinity. At this point both we buried beneath about 60 ft (18 rn) of Coastal Plain sediments. 01.15 0.55 Cross Richmond, Fredericksburg, and Potomac Railroad tracks and continue west on Va. Route 54. 01.2 0.05 At Ashland Town Hall bear right and continue west on Va. Route 54. 01.7 0.5 Bear right and continue west OR Va. Route 54. 03.6 1.9 Cross Falling Creek. This is the first point in our journey at which a stream has incised deeply enough to cut through the Coastal Plain section and exhume the underlying Triassic strat a. 03.8 0.2 Turn left onto Va. Route 666. 04.5 0.7 Turn right onto Va. Route 696. Somewhere dong this stretch of Va. Route 696 we cross from the Taylorsville basin back onto the Petersburg Granite. 05.3 0.8 Turn right into parking lot for Hanever County Dog Pound.
STOP 1: Doswell Formation, Stagg Creek Member and Falling Creek Member. Park and walk west (the same direction you were riding on Va. Route 696) about 100 ft (30 rn) to bridge aver Stagg Creek (fig. 7). On the near side Qf the bridge is a foot trail to the right which parallels the creek bank Figure 7.- Map showing detailed location of Stops I and 2 in the Yanover Academy 7.5-m i nut@guadr angle. downstream (north). Follow this path dong the edge of Stagg Creek, which flows ta your left across fault-sheared Petersburg Granite. At 900 ft (210 m), cross a small gully and begin ascending a steep ridge. At the base of the ridge, Petersburg Granite crops out 20 ft (6 rn) to your right. At 960 ft (29e m) along the trail, cross aver an outcrop of Petersburg Granite on the ridge. Trail crosses the top of the ridge, then descends ta 8: wooden foot bridge crossing a steepsided ravine at 1,100 Et (330 IT!). This gully roughIy marks the south border of the TayIorsville basin. IrnmediateIy beyond the bridge the trail forb bear left at trash barrel and stay beside creek. At 1,200 ft (360 m), trail skirts creek edge and Stagg Creek Member sandstone and conglomerate we visible along the creek floor. This material is a micaceous, very poorly sorted, silty sandstone with pebbles 0.2-2.4 in. (0.5 to 6.0 cm) in diameter scattered throughout. Pebbles are composed mostly of sheared and foliated Petersburg Granite and vein quartz. Because this material is so poorly sorted, it probably was deposited very rapidly, The closest source fer this material would have been from an upland developed immediately to the south, above the eroded stump of Petessburg Granite which we crossed to get here. Continue dong trail; massive to poorly bedded Stagg Creek Member is exposed intermittently along the stream bottom, especially around 1,550 ft (465 m). At 1,750 ft (525 rn) a Jmge outcrop of Stagg Creek sandstone and conglomerate is visible en the bluff that is near the creek on your right. It is similar to the previous outcrops except that the pebbles are slightly smaller (less than 1.6 in. or 4 cm). At 1,770 ft (530 m) the trail forks again, bear left and continue beside the creek. At 1,910 ft (573 m) pass a "NO Hunting" sign on tree to right of trail. At 1,960 ft (S88 m) the Stagg Creek Member again crops out in the creek bottom. It is poorly sorted, rn edium- to very coarse grained sandstone, with mica flakes to 1 mrn in diameter scattered throughout. It weathers into irregular humps and has no distinct bedding. At 2,080 ft (625 rn) cross a small gulley, and at 2,100 ft (630 m) the first exposure of well-bedded Falling Creek Member sandstone of the Doswell Formation crops out d~ngthe meek bed. This exposure is 8 moderately well sorted, fine-grained, light yellowish-brown silty sandstone bedded at 0.1-0.4 in. (0.5-1.0 ern) intervals; bedding planes are defined by an abundance of mica flakes. The bedding strikes N80'W and dips about 6OoN. At 2,150 ft (645 m) cross +
1 meter
pebbles
~1 medium- to coarse-grained -I 9 massive sandstone
fine-grained
pi fine-grained faintly ...... , --. ' planar bedded sandstone
fine-grained planar ... - . bedded sandstone
f ine-grained - - crossbedded sandstone
t-']--- siltstone
PI--- shaley siltstone 3 2 m share
crayey silt (underclay]
m coal
Firure 8. - Columnw section of CTutcrm A Rt Ston 1. another small gulley, and at 2,170 ft (651 m) come to the base of s second steep bluff. A good outcrop of the Falling Creek Member is present 70 ft 121 m) to your left where the creek impinges against the base of the bluff. This outcrop is shown diagramatically in figure 8 and is described as follows:
0 Outcrop A: Bedding strikes N75 W, dips 45 0N; section represents parts of generalized intervals 38-40 of section 1 in Weerns (1980b). Description of intervals begins at the top of the outcrop (see fig. 8).
Interval rn eters description 25 0.53 SILTSTONE, dark-ay to rnedium-gray, with 0.25 to 2.0 cm bedding, interbedded with SANDSTONE, finegrained, light- brown, with 0.5 to 2.5 cm bedding, flaggy, very micaceous, bedding planes defined by mica flakes; gradational contact with: 24 1.73 SANDSTONE, medium- to coarse-grained, light-brown, micaceous, massive, fines upward, scattered 2.5 cm diameter pebbles at base and sharp contact with bed below. 23 0.41 SANDSTONE, finegrained, light-brown, small mica flakes scattered throughout, poorly bedded; gradationd contact with: 22 0.33 SILTSTONE, light-bro wn, sandy, interbedded with SHALE, dark-gray, silty, with mrn to em bedding, well beddedr gradational contact with: 21 0.11 SANDSTONE , f i ne-grained, li gh t-bro wn, sIightly micaeeo us well cemented, with mm to ern bedding with mica flakes defining bedding planes; gradational contact with: 20 0.13 SILTSTONE, light-brown, sandy5 and SHALE , dark-gray, silty, interbedded at mm to em intervals, well bedded; gradational contact with: 19 0.28 SANDSTONE, fine-grained, light-brown, slightly micaceous, well cemented, mrn to cm bedding with mica flakes defining bedding planes; pdational contact with: 18 0.74 SILTSTONE, clayey, to SHALE, silty, dark-gmy, bedding poorly defined, fissile; gradational contact with: 17 0.03 COAL, black, greasy; gradationd contact with: lfi 0.02 SILT, light-gray and light-yellow mottled, cIayey, rn assive; "underclay"; gradational contact with: 15 0.53 SANDSTONE, finqrained, medium-brownisbgray, mnsive; gradational contact with: 14 0.31 SILTSTONE, dark-gray, bedding poorJy defined, blocky, includes a single 1 em-thick dark-gray clay layer; gradational contact with: 13 0.15 SANDSTONE, finegrained, dark-gray, silty, micaceous, massive; gradational contact with: 12 0.28 SILTSTONE, dark-gray, sandy, micaceous, massive soft; gradational contact wit fi: 11 0.31 SANDSTONE, finegrained, mediurn-gray, silty, micaceous, massive; gradational contact with: IO 0.53 SILTSTONE, dark-gray, bedding poorly defined, weakly fissile; gradational contact with: ! 0.25 SANDSTONE, fine-grained, m edium-brownish-gray, cross- bedded at cm scale, mica flakes define bedding planes; gradational contact with: 8 0.13 SILTSTONE, medium-brownish-gray, micaceous; gradational con tact with: 7 0.33 SA N DSTO NE f i negrained, rn edi u rn -bro wn, crossbedded with partings devdoped at crn scale, mica flakes define bedding planes; gradational contact with: 6 1.55 SILTSTONE, dark-gray, shaly, bedded at cm intervals, sIightIy bIocky; gradational contact with: 5 0.36 SHALE, black, very fissile, parts at rnm to cm intervals, pervasively slickensided; gradational contact with 4 0.53 SILTSTONE, dark-gray, shdy, parts at ern intervals, slightly blocky, moderately well banded; gradationd contact with: 3 11.15 SHALE, dark-gray to black, very fissile, parts at mm to cm intervah gradational contact with: 2 0.23 SANDSTONE, fin egrained, rn ediurn-gray , micaceous, rn i ca flakes define bedding planes, crossbedded with partings developed at mm scale; gradational contact with: 2.05 RESIDUUM, mostly developed from siltstone but includes possible bed of COAL 12 crn thick) and SANDSTONE (8-10 crn thick), poorly exposed. - 12.00 m Total thickness
Return to trail and ascend bluff. At 2,200 ft (660 rn) pass red barrel on right. Trail rises for a distance of 2,330 ft (700 m), then starts to descend. To Ieft is a ledge of Falling Creek sandstone (fine-grained, reddish-gray, well sorted, micaceous, crossbedded which probably holds up the crest of this bluff. At 2,440 ft (732 rn) pass mother ledge on your Ieft (coarse to very coarse grained, light-yellowisbbrown sandstone). More sandstone ledges crop out on left to 2,490 ft (747 m); these are coarse-grained. Descend bluff to creek bottom at 2,580 ft (774 m). Tree to left is hollow and has a beehive - PASS WITH CARE! From 2,580 to 2,910 ft (774 to 873 m) trail parallels a deep and murky stretch of Stagg Creek (Falling Creek Member, interval 59, in Section 1 of Weems, 1980b). This interval probably is composed mostly of soft shales and coals, as a cod mine once was located about 0.75 miIe (0.4 kml to the right along strike. At 2,910 ft (573 rn) traiI swings right around the head of a gulley and eldts at 2,960 ft (888 rn) onto a dirt road that descends from the hill on your right. To your left the creek again inpinges against the edge of its valley and exposes the. following section of the Falling Creek Member (see alsa figs. 7 and 9):
Outcrop B: Bedding strikes N80°E, dips 5OoN; section represents intervals 61-63 in PeneraIized Section 1 of Weems (1980b).
Interval meters description 16 0.3 I+ SILTSTONE, rnediurn-gray, Mocky, faintly bedded at mrn scale, very weathered at top; gradational contact with: 15 0.15 SANDSTONE, fine-grained, medium-brownisbgray, m &si ye, contains disseminated fine mica flakes; gradational contact with: 14 0.23 SILTSTONE, medium-browniskgray, soft, bedded at 0.25-0.50 crn intervals; gradational contact with: 13 0.03 COAL, Hack, shaley, fissile; gradational contact with: 12 0.02 CLAY, light-gray, unbedded; gradational contact with: 11 0.23 SILTSTONE, dark-brownish-gray, soft, faintly bedded at rn rn scale; gradational contact with: 10 0.15 SANDSTONE, very Sine grained, light-brawn, micaceous, poorly bedded; gradationd contact with: 9 0.13 SILTSTONE, Light-brownismay, sandy, micaceous, bIocky; gradational contact with: 8 0.94 SANDSTONE, fine grained, light- to medium-brownish-gray, bedded at 0.25-1.00 cm intervals, crossbedded; bottom bedding plane contains leaf impressions, some higher bedding p€anes have invertebrate trails; prominently joinkd with vertical offsets up to 8 cm; gradational contact with: 7 0.33 SANDSTONE, fine-grained, and SILTSTONE interbedded, m edium-browniskgray, bedding planar but faintly developed; gradational con tact with: 0.15 SANDSTONE, fine-grained, medium-brownisbgmy, massive, micaceous; gradational contact with: 0.99 SILTSTONE, ranges from d&Fk-gr&g, clayey and well bedded to rn ediurn-bro wnish-gray, faintly bedded and blocky; gradational contact with: 4 0.41 SANDSTONE, very fine grained, light-brownish-gray, massive, very finely micaceous; gradational contact with: 3 0.94 SILTSTONE, medium-brownismay to medium-ay, poorly bedded, blocky, very finely micaceous; gradational contact with: 2 2.46 SANDSTONE, fine-grained, IIght- to medi urn-browniskgray, crossbedded, parts mostly at 0.25-1.00 cm intervals but at Up to 8 crn intervals, micaceous, mica flakes define bedding planes; gradational contact with: 1 0.53 SILTSTONE, mediuIn-brownis~gray, sandy, micaceous, prominently bedded at 0.5-1.0 ern intervals, breaks into flawslabs, contains plant impressions.
8.00-c rn Total thickness f ine-grained E3.. , crossbedded sandstone
I.' 1 *. *.I fine-grained sandstone
very fine grained faintly planar bedded sandstone
very fine grained sandstone
well bedded siltstone
siltstone
coal
clay
Figure 9. - Columnar section of Outcrop B at Stop 1. Return to vehicle by the same path.
05.3 - Turn left onto Va. Route 696. 06.1 0.8 At stop sign turn left on Va. Route 666. 06.8 0.7 At stop sign turn left'on Va. Route 54. 07.6 0.8 Turn right on Va. Route 686. 08.5 0.9 Cross Stagg Creek. 08.6 OJ Pull over at near end of Horseshoe Bridge.
STOP 2: Doswell Formation, Newfound Menber, sandstone/siltstone facies. Park and cross road to left side of bridge abutment '(fig. 7). Descend slope of reddish-gray siltstone. At river go upstream 300 Et (LOO m) to exposure on left. This section is at the base of the sandstonelsiltstone facie of the Newfound Member of the Doswell Formation. The bmd bed in this area is the massive siltstone over which you descended at the south end of the bridge, The topmost bed of the sandstonelconglomerate facies of the Newfound Member is visible downstream beneath the bridge about 300 ft (I00 m) distant. The soft basal siltstone bed af the upper facies is apparently responsible for the small wind gap through which the road runs to the bridge. The following section is exposed (see also fig. 10):
0 Outcrop C: Bedding strikes N2S0E, dips 27 NW. Interval rn et ers description 3 fZ SANDSTONE, medium- to very coarse grained, light-bro wnish- gray, sparseIy micaceous, trough crosssbedded, contains clay lumps and rock pebbles to 3 crn in diameter, carbonized wood fragments, and rare bone impressions; channeled into bed below. 2 0.2-0.8 SANDSTONE, fine-grained, light-brownish-gray, cress-bedded, Iayess developed at mm scale, slightly micaceous, well sorted; gradational contact with: 1 *7 SILTSTONE, dark-reddiskgray, sandy, micaceous, blocky, burrowed and rooted.
49.5 rn Total thickness. a Y
Q a 1 meter m r a
2 Figure 10. - Uitigrainmatic portrnynl of Outcrop C at Stop 2. v1 08.6 - Continue wross Horseshoe Bridge over South Anna River. 10.3 1.7 Cross buried Fork Church fault and enter Hylas zone. 11.0 0.7 At stop sign turn right onto Va Route 795. 11.4 0.4 Bear left on Va. Route 686. 12.1 0.7 Come to Iarge pond on left; turn right onto Va. Route 685. 12.5 0.4 Cross Newfound River; to left are excellent exposures of Hylas zone mylonite gneiss. 13.3 0.8 At stop sign turn right on Va. Route 738. Fork Church is at this intersection, 13.6 0.3 Cross Fork Church fault and reenter TaylorsvilIe basin. 17.3 3.7 At stop sign cross W.S. Route 1. 18-0 0.7 Cross over Richmond, Fredericksburg, and Potomac BaiIroad Bridge, pull into gravel access road on right immediately past bridge; descend into railroad cut. WATCH OUT FOR TRAINS, OCCASIONALLY TWO ABREAST MAY COME FROM THE SAME DIRECTION AT THE SAME TIME.
STOP 3: Doswell Formation, Newfound Member, sandstoneJsiItstone facies. This outcrop (located on fig. 11) shows B massive sequence of trough crossbedded sandstones that appear to form fining-upward packages about 3-4 m thick. About 50 years ago, when this cut was made, numerous petrified logs (probably Araucarioxylon) were unearthed. Occasional pieces still are found. The textures in thk unit are typical of a well-developed fluvial system, being fairly coaFse.grained and also rather well sorted. This is considerably different from the poorly sorted, poorly bedded CengIomeratic sandstones of the Stagg Creek Member. The paleocurrent direction (northeast) is almost perpendicular to the outcrop trend. Most of the crossbeds are due to dunescale bedforms, but the large, more tabular sets near the tops of the sequences probably represent transverse bars formed during falling flood stages. This hypothesis is supported by the scatter in the dip orientations of these features.
Two diabase dikes, probably of Early Jurassic age, are present in the cut on the north side of the bridge over the railroad. The larger of these dikes can also be found along the south bank of the Little River to the Figure 11. - Map showing detailed location of Stops 3 and 4 in the Ashland 7.5-m inute quadrangle. northwest and dong the north bank of the South Anna River to the southeast (Weerns, 1985). These dikes are typical of those which have been found in the basin, and dl appear to postdate the time of deposition of the exposed strata in the basin.
The Doswell Formatian sandstones are capped by a thin sequence of gravel and overlying sand. These are Pliocene-Pleistocene in age and represent a fluvial terrace deposit formed by the ancestral North Anna River. Nearly all of the cobbles in these gravels are composed of quartz. Among these are sparse pieces of chert containing eoraIs which me derived from the Helderberg Group (Lower Devonian) and clasts' of quartzite containing tubes of Skolithos (Scolithus) which are apparently derived from the Erwin Quartzite (Lower Cambrian). Neither of these lithdogies crops out within or near the present drainage basins of the North Anna or South Anna rivers, so they were apparently introduced into the basin by Potomac River drainage and redistributed by longshore currents during a high stand of sea level in the Miocene when the eastern Piedmont was at or slightly below sea level. Contrast these lithologies with those which you wiIl see at Stop 8.
18.0 - Return toward U.S. Route 1. 18.7 0.7 At stop sign turn left onto U.S. Route 1. 19.4 0.7 Go down Tang hill and turn left into Riverview service station. To right of service station down hill is the bank of the South Anna River (see fig. 11).
STOP 4 (LUNCH STOP): The bluffs across the river are composed of the same Triassic beds as at Stop 3, and large-scale structures are more apparent from this distance. It iS suggested khat you contemplate their geometry while you eat.
19.4 - Load up and turn left (south) on U.S. Route 1. '19.5 0.1 Cross South Anna River. 20.0 II -5 Cross Falling Creek 20.4 0.4 Cross bridge over Richmond, Fredericksburg, and Potomac
Railroad. ' 22.8 2.4 Return to stop light and turn left on Va. Route 54. Leave TaylorsviUe basin. 23.4 0.6 Exit on Interstate 95 south.
End of detailed mileage log. Follow Interstate 95 south to the exit for Interstate 295 west and take. that exit toward Interstate 64 west and Charlottsville. Follow Interstate 295 west until it intersects with Interstate 64 and take Interstate 64 east toward Richmond. Get off Interstate 64 at the first exit (Short Pump), and turn right onto U.S. Route 250 heading west toward Short Pump. The detaiIed road log commences again at this intersection of Tnterstate 64 and US. Route 250.
rn ileage total increment 00.0 0.0 Proceed west on US. Route 250 from its intersection with Interstate 64. 00.7 0.7 Pass through the town of Short Pump, well known to fans of Jeff MacNellYs -Shoe. This we8 is underlain by the Petersburg Granite. 02.2 1.5 Turn left onto Va. Route 623, Gayton Road. 02.5 0.3 To the right is a good view from the higher land above the Petersburg Granite across the lower terrain of the more easily eroded sedimentary rocks of the Richmond basin. 03.9 1.4 Cross over the contact between the Petersburg Granite to the east and the lower barren beds and productive coal measures of the Richmond basin to the west. Although the contact has not been directly observed in this pest of the basin's eastern margin, it appears to be a nonconformity, along which the top of the granite and overlying sedimentary rocks dip westward at angles averaging 20 to 35 degrees. Open pit mines and prospect pits were once common dong the exposed CQ~seams, and waste material from the mal mines is abundant in the soils of this area. Several mines formerly were operated on both sides of Gayton Road south of this point and this region was a major coal-mining center of the Richmond basin. 04.1 0.2 Continue straight onto LauderdaIe Drive. This road is on the western edge of the area dominated by formerly active coal pits and shafts. However, as mining followed down the dip of the coal beds, much of the developed area here overlies old mine workings. For example, one drill hole penetrated about 90 feet of sandstone and shale before terminating in 15 feet of void. The need for careful site investigation before construction in this area is apparent. 05.7 1.8 Cross creek. Mine waste constitutes the banks and contaminates the soil on the east side of the road. Mines were located on both sides of the road in this vicinity. 05.9 0.2 The eastern margin of the Richmond basin is crossed in this area. 07.3 1.4 Turn right onto Va. Route 6, Patterson Avenue, and proceed west. 07.4 0.1 To the north BCPOSS the parking lot and on a west-facing dope are large: exposures of relatively fresh, spheroidally Weathered Petersburg Granite. Such exposures and rounded bodders of granite often occur near the contact with Triassic sedimentary rocks and signal proemity to the contact. 01.5 0.1 When the parking lot to the north was being built, the eastern rnargh of the Richmond basin was exposed. It appeared to be a nonconformable contact, and the Triassic rocks dipped westward at 30 tu 40 degrees. The Richmond basin underLies our traverse from this point westward until beyond Manakin. 08.7 1.2 During widening of Va. Route 6, good exposures of the Vinita Beds temporarily existed in cuts QII the south side of the road. On the west slope of the hill, a small exposure of pay siltstone and finegrained sandstone still remains. Some of these beds yielded fish and plant fossils, and this was one of the palynological localities (VN 1) for Bruce Cornet's (1977) studies. 11.9 3.2 Pass through the town of Manakfn. This was an old cod mining center on the western margin of the Xichrnond basin, and mines shafts were situated both north and south of Va. Route 6. 12.5 0.6 Turn left across the center island and onto the road leading into the BoscobeI Quarry. As YQU go down this road there is a good view southward across the valley and floodplain of the James River. 12.9 0.4 Check in at Luck Stone office to obtain permission to enter quarry. 13.2 0.3 After bearing right toward the quarry at the base of the hill, pull off to the right by the high cut banks in sandstone and shale. Beyond these cuts is the deep Boseobel Quarry.
STOP 5: Petersburg Granite and Tuckahoe Group, Lower Barren Beds (BoscobeI Quarry). This deep quarry at the edge of the floodplain of the James Biver (fig. 12) was opened in 1924, and rock has been quarried from this general location for more than 125 years {Gooch, Wood, and Parrott, 1960). This quarry is deveioped within a small, wedg-haped horst of granite which is bounded to the east and west by Triassic sedimentary rocks. On the north wall of the quarry, more than 50 ft (IS rn) of saprolite showing 8 spectacular soil profile overlies unweathered granite; this overburden thickens northward under the adjacent uplands. The deep weathering of the eastern Virginia Piedmont and consequent devehpment of thick saproIite, residuum, and soiI has greatly reduced the chances of finding fresh bedrock near the surface. As a result, exposures of bedrock are scarce and almost invariably strongly weathered. Most surficial geological studies in this area must be made on saprolite or residuum, and even moderately good exposures of Triassic rocks usually are found only along streams.
A normal fault at the east end of the quarry separates granite from the Triassic sedimentary rocks of the Richmond basin. Quarrying operations have exposed this fault to varying degrees at differing depths and angles over time. Measurements of the fault attitude have varied accordingly, and have ranged from NIOOW, 57% CGooch, Wood, and Parrott, 1960) to N-S, 65'E (Goodwin, 1970). A gouge zone 8s much as B in. (15 cm) thick has been observed along this fault. This fault is not the major western border fault of the Richmond basin (Goodwin, 1970; Goodwin and Farrell, 1979) because neither the Hylas zone cataclastics nor a coarse boulder breccia are present. The main border fadt QCCU~Sto the west (fig. WATERNARY ALLUVIUM STRIKE & DE OF BEDDING Nl COAL MINE PIT
COAI; MINE SHAFT
CRYSTALLINE EDCKS
Figure 12. - Geologic map of Stop 5 and vicinity (gedogy modified from Goodwin, 1970). Cod mines located by G. Wilkes. 31, and the fault seen here bounds the eastern side of a wedgeshaped horst of granite which was uplifted after deposition-of the Triassic sediments. Postdepositional uplift and drag on this fault has caused the Triassic rocks to the east of the BoscobeI quarry to dip steeply eastward, thereby giving the portion of the Richmond basin dong the James River a general synclinal shape between the Boscobel Quarry and the east margin of the basin. Sedimentary rocks elsewhere within the basin, both to the north and south of the James River section, dip regionally westward (Goodwin, 1970; Gmdwin and Earrell, 1979). The exact amount of displacement along this fault is unknown, but it is probably less than 500 ft (150 m); Some of the beds next to the fault stand nearly vertical in asymmetric folds. NEW the fault, the sedimentary rocks are intensely folded and shales have thickened and thinned dramatically. Much movement has taken place within the shales, which often show smooth, polished shear surfaces and slickensides. Folding diminishes greatly within 50 ft (15 rn) of the contact, but farther away numerous subsidiary normal faults with as much as 10 ft (3 m) of displacement cut the sedimentary rocks, forming small fault-bounded blocks. These faults commonly have dips of approximately 60 degrees, and beds show 8 normal displacement along the faults. Deformation decreases eastward from this quarry until the rocks appear to be relatively undeformed. PossiMy more deformation has occurred there than can be recognized from study of the sparse outcrops.
Because of their pcoldmity to granitic socks, these sedimentary roeks presumably are part of the Iower barren beds (Shaler and Woodworth, 1899; Goodwin, 19701, but LithoIogically they are indistinguishable from the Vinita Beds. Because the thickness of the Triassic section beneath the quarry floor is unknown, a definitive stratigraphic assignment is presently impossible. A detailed stratigaphie section is not given here because this rock is quarried occasionally and the visible section changes 8s rock is removed, but a generalized stratigraphic column is shown in figure 13.
Dominant lithologia are rnediurn- to dark-gmy, fine to coarse- grained sandstones, and black shales which commonly show no internal structure except for the abundant pinch and swell laminae. The friable sandstone weathers to a mediurn-brown color; it is imrnature mineralogically, containing much feldspar and muscovite. Fragments of SA.KDSTWNE, FINE- TD,VERY CDARSE-GRAINED, CONGLOMERATIC IN PLACES, MASSIVE WITH SOME GRADED BEDDING, SOME THIN BLACK SHALE INTPBEDS.
SWE, BLACK, CARBONACWUS, MICACEDUS, 5oPE FDE3-GRADlEE SANDSTONE BEDS UP 20 TWO DIMES THICK WITH SHARP CONTACTS,
SANDSTONE, VERY FINE- M MEDDJM-GRAINED WITH MODEXATE SORTING, MICACEDUS, GRAY, SOME GRADED BEDDING Dl BEDS UP M 18 INCHES WICK, SHALE PARTINGS UP M 4 INCH mZM.
S'XALE, BLACK, CARBONACEOUS, MICACEOUS, SOME FllE-GI1AINED SANDSTONE 3ms UP m TWO lXCKE3 TRICK WITH SHAW 03 NTACTs .
SANDSMNE, WRY M rnARSE-GRAINED, MODERATE SORTING, MICACEDUS, GRAY, WELL JOINTED, SOME GRADED BEDDING. SHALE, BLACK, CAABONACEDUS, MfCACM)US, %ME FINE-GRAINEU SANJETONE BEDS UP To TWO DICIES THICK WIT9 SHARP CONTACTS, HIGHLY DEFORMED, MNTACT WITH UPPER SANDSMrn SHAFS. SANDSTONX, VERY FW- To MEDIUM GRAINED, POOR SORTING, MICACEOUS, GRAY, KIP FTkt FEET INTEXBEDDED WITH BLAM SHALES UP To -FOUR 3NCHES THICK.
. ..
SHALE I BLACX , CARBONACZDUS, MICACEDUS , SOME FINE-GRAINED SANDSTONE BEDS. I-I C3VELPEDm
Figure 13. - Generalized geofogic section of the Triassic rocks exposed in the Boscobel quarry. plant fossils and fish scales are locally abundant; Schaeffer and McDonald (1978) and Olsen, McCune, and Thompson (1982)have described fossils of the fishes Dictyopyge mn8crwus and Cionichythys &from this locality.
Exposures away from the faulted contact with the granite show at least two and probably four cycIic sfternations of black shale and sandstone which appear to coarsen upward gradually. At the base of each sequence is a 3-5 ft (0.9-1.5 m)-thick bIack shale which in turn is overlain and cut into by 3-6 ft (0.9-1.8 rnkthick, massive, cmrsegrahed sandstones. The basal black shales in each sequence are badly sheared and show little or no internal structure. Where they approach the first thin sandstone bed they have pinch-and-swelI sand laminae which probably represent poorly developed osciuatory ripples.
The thick sandstone beds in the interbedded sandstone and shale zones have sharp bases with soft sediment load-casts. These sandstones appear to be graded, although the upper contacts have an abrupt transition to the next overlying shale bed. The thinner sandstone beds in the same interbedded zones me internally massive or have flat laminations; the upper parts frequently have a wavy character resembling ripple troughs which, because of the low-angle nature of the internal laminae, may be oscillatory ripples. The planar lamination and ripple troughs are best defined where these thin beds are thickest. These thin kds also may show gently inclined sets of internal laminations which laterally pinch and swell. These roughly resemble hummocky cross-strata, although the internal laminae are less well defined and the characteristic cenvex-up draping over scorn is absent.
The uppermost massive sandstones have scour bases and abundant mud intraclasts. The Iower parts have little or no internal structure but the upper parts have flat or hummock-like laminations. Thin shaly interbeds contain abundant flat shale clasts and carbonized wood f ragm ents .
It appears that the sands in this sequence were introduced rapidly because they are graded, poorly sorted, texturally immature, and contain load casts. Turbidity currents, flash floods, or storm-wave activity are possible causes of this type of depQSitiQIl. There is no apparent evidence of subaerial conditions in the form of root casts or mud cracks. If properly identified, the mcillatory ripples indicate same wave action, but the abundance of shale indicates mostly quiet water so this was not a beach environment. There is no direct evidence of fluvial action and no beds of unidirectional flow ripp€e cross-lamination have been identified as yet, though possibly some of the beds of flat lamination could have been formed by unidirectional current or turbidites. Mudstone intraclasts in the sandstone beds could have been derived from the erosion of dessicated muds, but channdling of wet cohesive material is more likely. The uncommon sediment-filled tubes me probably burrows, and some of the more diffuse bedding could reflect intense bioturbation.
13.9 0.7 Retrace route to the intersection of the road from Boscobel Quarry with Va. Route 6* Turn right (east) onto Va. Route 6. L5.4 1.5 Bear right onto Va. Route 650, River Fload. 17.8 2.4 Turn right onto dirt road by sign for Tuckahoe Plantation. At the end of this road on a bluff overlooking the Jmes River valley is Tuckahoe Plantation which was begun in 1715. Thomas Jefferson studied in a little schoolhouse OR the plantation grounds before going to the College of WilIiarn and Mary to complete his higher education. 13.5 0.7 Turn right into the cleared area designated for parking and park there by the trees at the western edge of the area. Walk west through the woods to the intermittent stream bed.
STOP 6A: Chesterfield Group, Vinita Beds (sandstones, siltstones, and shaIes):
Sandstones, siltstones and shales of the Vinita Beds are present in the bed of the stream and in a small tributary joining the stream from the northeast (fig. 14). Neither the stream nor the tributary are shown on the topographic map, but the small valley which they occupy and B pond into which they empty can be seen. The exposures are small and weathered, hut are comparatively good for this area. As is commonly the case, the more resistant sandstones form streambd expmures whereas the softer shales tend to be eroded away and rarely form good exposures. Thus, examination I I 0 [OOOm
r44 TERTIARY I,Tgbl GRAVEL 7 STRIKE DIT OF BEDDING
x COAL NINE PIT
Figure 14. - Geologic map of Stop 6 and vicinity (geology modified from Goodwin, 1970). Coal mines located by G. WiIkes. of stream exposures usually gives a biased impression of the relative abundance of sandstones vemx shales. Heinrich's (1878) section showed 57 percent finegrained sandstones and 43 percent dark-gray to black, often fossiliferous shales in the central part of the Vinita Beds. At the top of Heinrich's section, gray, finegrained, arkosic sandstones dominate and make up 80 percent or more of the rocks. fn these streams, a sandstone- siltstoneshale sequence can be seen, whereas adjacent to the James River at Stop 6B the exposures are primarily of massive sandstones.
The Vinita Beds here are overlain by nearly horizontal Tertiary grave& at an elevation of about 190 ft (57 m). In both the main stream and the tributary B sharp break in gradient and in depth of the stream bed is found at the Triassic rock-Tertiary gravel contact. Bedding in the Triassic rocks is nearly horizontal, and the sandstones are very welt jointed with two prominent joint sets nearly at right angles, This site is another of Cornet's (1977) &nological localities IVBA4) where he obtained his specimens from "... a thin black clayey siltstone within a brown to reddish brown sandstone and conglomeratic sandstone ..." He also reported that fossil fishes are found here. A measured section follows. It begm st the first exposure upstream from the pond, follows the stream's bed upstream until the tributary, md then continues up tile tributary. The base of the section is nearest the pond.
--Feet Inches Description 0 3 SANDSTONE, bluegray, very fine to mediurn-grained, micaceous with minor pyrite. 0 S SHALE, blue-gray, silty, minor muscovite. 0 3 SANDSTONE, light-bluegray, very fine to rnedium-grained; breaks along muscovite-rich thin zones. (Tributaryl 1 0 SHALE, bluegray, silty, minor muscovite, Fades up into: I G SANDSTOWE, light-gray, rn ediurn-grained, minor muscovite 1 il (Covered) 0 6 SANDSTONE, light-gray, very fine to cwrse-grained, poorly sorted; iron occurs as grain coatings, dirty. 4 0 (Covered) 3 10 SANDSTONE, light-pay, very fine to medium-grained, noderately sorted; trace of muscovite. 1 0 (Covered) 0 2 SANDSTONE, light-gray, very fine grained, wen sorted, silty. 0 2 STLTSTO NE, light-gray . 0 2 (Covered) 2 10 SANDSTONE, light-gray, very fine grained, moderateIy sorted, beds as much as 6 in. (15 cm) thick; slightly micaceous. 0 5 SANDSTONE, light-gray, very fine to coarse-grained, poorly sorted, silty 0 11 SANDSTONE, light-gray, very fine to comsegrained, moderate sorting, trace of muscovite. 2 0 (Covered) 2 10 SHALE, bluegray, silty, minor muscovite. 4 0 (Covered) (ANGULAR UNCONFORMITY1 Tertiary gr%vels
27 ft 4 in Totd thickness
Return to vehicle and exit parking area. Turn right on dirt road at exit of parking area. 18.6 0.1 At "T" in road turn left in front of buildings and follow dirt road east. 18.7 0.1 Pass through gate in fence and then bear to the right and park in this area. Leave vehicle here and walk south and downhill on the middle fork of the dirt road to the left of the corn crib. Follow this road €or about 0.2 mile to the railroad tracks.
STOP 6B: Chesterfield Grow. Vinita Beds (massive sandstones): On the north side of the road and up the small valley to the north me exposures of massive sandstone (fig. 14). Similar sandstone forms the bed of the road uphill. The sandstone is gray, coarse to very coarse grained, and thick bedded with beds as much as 2 feet (0.6 m) thick. It is micaceous and the grains are rounded. Same of the coarsegrained beds are crossbedded and sandstones dominate the exposures. A few hundred feet to the west along the bIuffs is a small, long abandoned quarry in which a 15 ft (4.6 m)-high wall is composed almost entirely of similar sandstone. The floor of the quarry is covered with water so it cannot be visited at this time. The quarry provided stone which was used in some of the steps at Tuckahoe Plantation. It is &o reputed to have served as 8 turn-around point for barges on the canal which is at the base of the slope and parallels the James River.
19.6 0.9 Go back to vehicles, board, and retrace route back to Va. Route 650 (River Road). Turn right (east) on Va. Route 650. 21.2 1.6 Cross over the approximate contact between the Richmond basin to the west and the Petersburg Granite to the east. 25.8 4.6 Entrance to the University of Richmond is on the left. 26.5 0.7 Turn hard right onto Va. Route 147 (Huguenot Road). You are now on the floodplain of the James River and, after passing over the canal, you will cross the James River. This bridge is at the western edge of the Fdl Zone and rapids and exposures of Petersburg Granite are found in the bed of the river eastward to the point where Interstate 95 crosses the James River. 28.8 2.3 Cross over the contact between the Petersburg Granite and higblevel gravels of probable Tertiary age. The base of the gravels is subhorizontd, gently inclined eastward at about 7 feet per mile, and iS commonly marked by a sharp break in slope. These gravels underUe the broad, relatively flat, upland surf ace ahead. 33.5 4.7 Turn right onto Va. Route 677 (Old Buckingham Road). Tertiary gravels have underlain the traverse for the past 4.7 rn iles , 34.4 0.9 Turn right onto Old Cod Mine Road. 34.8 0.4 Turn right on Deerhurst Drive. 35.2 0.4 Park in cul-de-sac at eastern end of Deerhurst Drive. STOP 7: Tuckahoe Group productive coal measures: At this locality (see fig. 15) the productive coal measures are found within a structural trough or subbasin called the Black Heath basin. Adjoining it to the west is the smaller Cunliffe basin which in turn is adjacent to the main Richmond basin. These two basins are part of the main Richmond basin and me in a downfalilted extension of it. They are on strike with similar basins, the Stonehenge and Union basins, to the south. However, the Stonehenge and Union basins, which also yielded cod, are completely separated from the Richmond basin proper and are enclosed by Petemburg Granite. The relationship between these basins and the flexures or "troubles" found down the dip of coal beds in many mines on the eastern margin of the Richmond basin is shown in the cartoon in figure 16. Qn that sketch map the main coal exposure is shown and the inferred geometry at depth is shown in cross sections A-A' of the main basin margin, B-B' of the Black Heath area, and C-C' of the Stonehenge and Union basins. This interpretation suggests that the main difference between these areas is in the relative depth of erosion.
Four coal seams were reported in the Midlothian area. At the Black Heath basin, which is just north of Midlothian proper, the lower two coal seams were considered to farm a single coal unit 40 ft (12 m) thick which included 15 ft (4.6 m) of parting material. The upper two coal seams &re 1 ft (0.3 m) thick and 5 Et (1.5 m) thick. The top seam contains 1.5 ft (0.4 m) of parting material. An analysis of the 40 ft (12 m) thick Black Heath coal by Johnson (1846) showed it to be high volatile A bituminous in rank. Sulfur w&s less than 2 percent and ash content WHS greater than 8 percent. Methane was present in the coals of this area, for Wooldridge (1842) wrote that: "Large quantities of inflammable gas are thrown out from the Coal in the mines constantly...".
The Black Heath Mine, located near the center of the Black Heath basin, was opened around 1788 by the Heath Mining Company. It began With an 800 ft (240 m) deep shaft mine which was supported by wooden cribbing. Coal, personnel, and equipment were raised and lowered by buckets hung from a wooden head frame, and a mddriven windlass drove the buckets. The 40 ft (12 m)-thick Black Zeath coal was penetrated at the jp:zi;_bj PETE3SBURG GFLANITE aAL MINE PIT
COALMWESHAFT
Figure 15, - Geologic flap of Stop 7 and vicinity (geology modified from Goodwin, 1970). CQ~Zmines located by G. Wilkes. A
Figure 16. - Sketch map of the exposed coal beds and basin margins near Midlothian, and hypothetical cross sections across the basin mmgins'(geo1ogy modified horn Heinrich, 1873; Shaler and Woodworth, 1S99; and Goodwin, 39701. bottom of the shaft, and the main gangway developed in a westerly direction for 1,350 ft (412 rn] and dipped from 8-to 42 degrees. Galleries or rooms were driven off the main gangway in haphazard manner, depending on coal thickness and structure, The coal was first undercut by pick and then several I in. (2.5 cm) holes were drilled into the coal face. The holes were then loaded with black powder which was set off with a fuse. The fallen coal then was loaded into carts and pulled by mdes to the base of the shaft. Roof support was added as needed by wood timbers and cribbing, which was in ample supply from the adjacent forests.
By 1836, part of the workings were on fire but had been sealed off from active sections. A unique ventilation system was developed by using this abandoned burning section. A rnandoor, located in the stopping separating the active and abandoned worldngs, was occasionally opened to suck fresh air into the mine at the portal by the natural tendency of air to be drawn into 8 damped fire. Care had to be exercised because flammable concentrations of methane also could be drawn into the fire. In 1839 this did happen, and a violent gas explosion occurred which threw three rnen in a basket, which was descending the shaft at the time, over a hundred feet into the air. Fifty-three men in the mine were killed, and the underground workings could be outfined on the surface due to the coUapse of sections by the explosion.
Sir Charles LyeU descended the Black Heath Shaft in 1344 and pubIished his view on the coal and surrounding stata (Lyell, 1847). He visited the workings just in time, because later that year a gas explosion kiUed eleven men. The mine was still active in the 18507s, but all minable coal in the BIack Heath and Cunliffe basins was mined out by the early 1860's. All that remained of the Black Heath Mine by 1887 was a pond. This pond was fiIled in 1985 (too late to stop E. 1. Robbins from falling into it) with tree trunks and brush from IQC& urban devebprnent as a prelude to the obliteration of the remaining traces of mining activity in this area. When Lyell entered the Black Heath Mine, he noted "-..a chamber more than 40 feet high, caused by the removal of coa1." As the workings progressed, the dip of the coal bed sometimes increased to as much as 84 degrees. In these steeply dipping sections, the coal bed gained more parting materid. Cod thickness diminishes away from the center of the bash and the marginal coals are heavily slickensided. At upward flexures, the coals are thin and of poor quality.
The stratigraphy and characteristics of these coals suggest deposition and lithification in 8 tectonically unstable area. As the middle Carnian land surface subsided, sediments poured in and filled the lowlands. Favorable dim ate, surficial geography, and flora contributed to formation of peat swamps (Xobbins and others, in press), whik occasional lowering of the land surface or runoff from starms introduced silt or sand to the swamp. Eventually, the swamps were covered by sands, silts, and clays and lithification of peat to coal began. Probably this area was still tectonically active, thus affecting the lithification process and distorting
the less competent beds such &s the coals. The coals found in the trough of a flexure (or of a basin such as the Hack Heath basin) are blocky, bright, and attain their maximum thickness, whereas near or at the top of flexures they become part of the distorted structural framework,
From the parking place at the cul-desac on Deeshurst Drive, walk north through the woods, over the tracks of the Southern Railroad, and down the embankment to Black Heath Creek. The large boulders in and near the creek are of a highly porphyritic phase of the Petersburg Granite. Between the smite and the first exposure of Triassic sedimentary rocks, you wiU walk over the nonconformity between the two units. Unfortunately, it is entirely concealed in this area. Continue waking downstream, taking note of the sedimentary rock debris of sandstone, shale, and cod in the creek. Begin the following measured section in the creek at the first exposure you find. The section follows along Black Heath Creek going downstream. During high water stages this section will be almost totally covered and not available for study.
Strike Thickness Lithology Description and diE (in feet) NlO'E, 25'W 1.0 COAL Well developed medium cleat, face cleat: N17'W, 67OSW; butt cleat: NTO'E, 7?OSEm 1.5 MUDSTONE Light-gray, plastic, plant fragments 2.7 SHALE Micaceous, medium-gray, one 3-in. thick CARBONACEOUS SHALE bed. 9.0 SILTSTONE Medium-pay, micaceous 5.0 SANDSTONE Mediurn-gray, very fine to coarse grained, rounded, poorly sorted, with minor CLAYSTONE interbeds. 1.0 SANDSTONE Dark-gray, poorly sorted, abundant clay, micaceous, undulating contact with overlying sandstone. 0.2 COAL Poorly developed cleat, dirty. 0.1 MUDSTONE Medium-gray, coal laminations, plant f ragm ents . 0.5 COAL Moderately developed medium cleat, includes 0.1 ft interbedded parting material, two sets of face cleats: N80°W,85'NE and N60°W, 87ONE. 0.2 MUDSTONE Light-gray, plastic, micaceous. V47'E, 20'NW 0.5 COAL Moderately developed fine dear, top of coal is covered. 5.0 (COVERED) 20.0 SANDSTONE Medium-gray, very fine to rnedium- grained, poorly sorted, dirty, with interbeds of SILTSTONE and MUDSTONE, grsdes into overlying rn udstone. 2.0 MUDSTONE Medium-gray, silty, micaceous. 0.2 COAL Incomplete section 15.0 (COVERED) BLACK HEATH POND N29'E, 22'NW 3.0 SILTSTONE Brownlgmy, dirty, incomplete section. 4.0 (COVERED) 5.0 SILTSTONE Browrdgray, bedding 2.5 ft thick. 0.1 MUDSTDXE Light-gray, plastic. N35'E, 35%W 20.0 SANDSTONE Gray, grades from poorly sorted, very fine to coarsegained with MUDSTONE interbeds at top to CONGLQUERATIC SANDSTONE with 0.25 in clasts at the base, joint xts: N5'E, 81%V and ?T4Q0Y, 84'N.E. 26.0 Si 1,'rSTONE Partially covered, AND mediutn-gray, micaceous MUDSTONE, indudes minor interbeds INTERBEDDED of poorly sorted SANDSTONE and one 0.5 ft thick COAL. N50°E, 26'NW 4.5 SANDSTONE Dirty brown, very fine to medium- grained, trace of conglomerate, moderate sorting, indurated; joint sets: N7Q0W,650SW and N27'E, 45'SE. N4O0E, 28'NW 8.0 SILTSTONE With minor SANDSTONE AND stringers and a 0.3 ft MUDSTONE foot thick CARBONACEOUS INTERBEDDED SHALE/COAL interval. 5.0 (COVERED) 3.0 SANDSTONE Gray, very fine to rnediurn-gmined, moderate sorting, micaceous, 20.0 (COVERED) 11.0 SANDSTONE Gray, very fine to medium-grained, moderate sorting, micaceous, indurated, with minor CLAYSTONE interbeds; joints: N5'W, SOONE. 0.8 UNDERCLXY Light-gray, plastic, rooted. N35'E,3Z0N W 0.6 COAL W eu-developed medium cleat, moderate thin lamination, face &at: N89'W7 73%; butt cleat: N17%, 65% W. 3.0 MUDSTONE Gray, plant fragments, minor SILTSTONE lenses. 10.0 SANDSTONE Gray, CQBTS~to very coarse grained, well sorted, micaceous, indurated. 10.5 SANDSTONE Gray, fine to very coarse grained, moderate surting, micaceous, with MUDSTONE interbeds up to 1.5 ft thick. 1.5 UNDERCLAY Sandy, rnedium-gray, rooted.
N20°E, 27'UW 0.7 COAL Well-developed medium cleat, bright, lowest 0.1 ft slickensided, face cleat: N39'W, 76'NE; butt cleat: HIS%, .73%w. 1.5 MUDSTONE Gray l*O SILTSTONE Sandy, gray, incomplete. GAS PIPELINE - END OF SECTION
203.1 ft Total thickness
35.2 0.0 Turn around and retrace route west on Deerhurst Drive. 35.4 0.2 Turn left onto Black Heath Road. 35.8 0.4 Turn left onto Va. Route 677 (Old Buckingharn Road). 36.6 0.8 Turn right onto Va. Route 141 (Huguenot Road). 36.1 0.1 Turn right at traffic fight onto U.S. Route 60 (Midlothian Turnpike). The high level Tertiary gravels are weU exposed in some of the construction &res on the right side of the mad. 37.3 0.6 Cross the contact between overlying Tertiary gravels and the Petersburg Granite. This contact was once well exposed at the western end of the former gravel pit to the right. 37.6 0.3 In the small roadcut on the right side of the road is the contact between the Petersburg Granite and Triassic sandstone, shale, and cad. The Trimsic racks here occur
within the Union basin (fig, 161, a small outlying basin tQ the east of the main Richmond basin. Immediately north of the road are waste piles from the old coal mines and remnants of the former mining operation. Granite lies between the Union basin and the Richmond basin. 37.9 0.3 Cross over the approximate contact between Petersburg Granite and the Richmond basin. 38 .o 0.1 Cross over the contact between Triassic sedimentary rocks of the Xichmond basin and overlying, nearly horizontal Tertiary gravels, These gravels underlie a flat upland surface dong U.S. Route 60 for the next 4 miles to the west. 38.5 0.5 Pass through the center of the town of Midlothian. This was an active coal mining center in former years, and mines were situated bath north and south of US. Route 60. A few Qf the old shafts can still be seen, although they me rapidIy being destroyed and covered by urbanization. 40.1 1.6 Turn left onto Va. Route 667 (Otterdale Road) and proceed south. 40.3 0.2 Cross the tracks of the Norfolk-Southern Railway. Good exposures of Tertiary gravels QCCW in cuts along the railroad tracks to the west of this intersection, 41.2 0.9 Cross over the approximate contact between Tertiary gravels and Triassic sedimentary rocks. 42.7 1.5 Cross Va. Route 652 at stop sign. Continue south on OtterdaIe Road. 44.3 1.6 Cross Swift Creek with its broad floodplain. The larger streams have commonly developed broad floodplains over the easily eroded sedimentary rocks of the Richmond basin. 44.6 0.3 Weathered exposures of the Vinita Beds occur on the left side of the road in a high roadcut. 44.9 0.3 Cross Va. Route 604 (Genito Road). Continue south on Otterdale Road. 45.6 0.7 Cross Otterdale Branch where conglomerate is exposed in the bed of the creek on the right side of the road. 46.5 0.9 Three wells were drilled around 1983 by Merrill Natural Resources between this road and the floodplain of Deep Creek to the east.
47.6 1.1 At "T" junction, turn left and continue to the southeast on Va. Route 667 (Otterdale Road). 49.4 1.8 Turn right on US. Route 360 (Hull Street Road) and proceed west. SQ.6 1.2 Turn €eft onto Va. Route 730 (Baldwin Creek Road). 51.9 1.3 Park near intersection of Baldwin Creek Road and Va. Route 655 (Beach Road). STOP 8: Chesterfield Group, Otterdale Sandstone: A series -sf $-foot high exposures in the roadcut on the west side of Va. Route 730 (fig. 17) are developed within residuum of the Otterdale Sandstone. Although these expmmes would be considered to be small in many areas, for the Richmond basin they are superb. Quartz grains remain, but.feldspars mostly have been converted to clay. This has caused many of the former rocks to thoroughly decompose. Although quartz cIasts remains hard and relatively fresh, schist, phyllite, granite, and sedimentary rock have been thoroughly altered to sandy clay and usually are recognizable only as relicts of former clasts. - One problem of geologic mapping in this area is in distinguishing residuum of conglomeratic parts of the OtterdaIe Sandstone from residuum of upland gravels of Miocene(?) to Pleistocene age which occur as nearly horizontal sheets on many upland surfaces. The Tertiary upland navels have also been deeply weathered and have pebbles and cobbles similar in size to those of the Otterdale Sandstone. However, the basal contact of the Tertiary upland gravels occups at quite predictabIe elevations (except at the base of channels cut deeply into the underlying rocks) OR a surface that is inclined gently eastward dong a gradient averaging 7 ftJrni (1.3 m/krn). It reaches a minimum elevation of about 250 ft (75 m) at a scarp which commonly marb the eastern limit of this gravel unit south of Richmond. In the vicinity of this locality, the basal contact of the Tertiary gravels should be at an elevation of about 330-350 ft (100-106 m), if they once were present. The exposures here are at 270 ft (8lm), 8. much lower elevation. Also, 8 short distance north in the upland east if State Road 730 is an old coal mine (fig. 17) which exploited a 5 ft (I.5m) seam of coal at an unknown, but presumably shallow, depth. No detailed comparison of clast composition has been made between the Triassic congIomerates and the Tertiary gravels, but it appears that the Triassic rocks have many more clasts derived from igneous, metamorphic, and catadastic rocks than the Tertiary gravels, which appear to be much richer in quartz, sandstone, and quartzite clasts. Some quartzite clasts in the Tertiary gravels bear tubes of the fossil Skdithos (Scolithus). These have never been found IOOOrn PI DIABASE .% NORMAL FAILLQ
I VLNITA BEDS )(" COAL ME@, PIT COAL MEASURES AND 3ARREN BE3S @ COAI, MINESHAFT
Figure 17. - Geologic nap of Stop 8 and vicinity (geology rnodilied frm Goodwin and Farrell, 1979). Cod mines located by G. Wilkes. in the conglomerates of the Richmond or TaylorsviIIe basins or in Coastal Plain sediments older than Miocene. This suggests that the pebbles and cobbles in the Triassic conglomerates were derived mainIy by erosion of nearby Piedmont areas adjacent to the Richmond basin, whereas many of the similar-sized clasts in the Tertiary gravels were transported much farther, some Srom beyond the crest of the Blue Ridge. The clasts in the congIomerate and conglomeratic sandstone at this locality reflect the minerdogicahfv more heterogeneous mmpositions characteristic of Triassic rather than Tertiary sediments. Thin conglomerates also are found within the Vinita Beds, and fanglomerates are found IocaUy along bath the eastern and western margins of the Richmond basin.
As no diagnostic fossils have been found as yet within the Otterdale Sandstone, except for common petrified logs and tree branches, these rocks have always been assumed to be Triassic in a~e.However, this age was based more on the stratigraphic position of the Otterdale than on hard evidence. If, as suggested by some drill hole correlations, the Otterdde Sandstone and the Vinita Beds interfinger, then the Otterdale must necessarily be Carnian in age. However, if the Vinita Beds and the Otterdale Sandstone are separated by Bn uncclnforrnity as Cornet (1985) has suggested, then the Otterdale might be younger than Carnian in age. The petrified logs of AraucarioxyIon, one of which can be seen at this locality, are found in roc& of Triassic and Jurassic age but have not been reported from rocks of Cretaceous age. A more exact paleontological age assignment for the Otterdale must await further study. However, the fact that diabase dikes of presumed Early Jurassic age cut across the Otterdale Sandstone, and no basaltic flows have been reported within it, ten& to suggest that the Otterdale is older than Jurassic in age.
These exposures me developed in what was once medium- to coarse grained, arkask sandstone, pebbly sandstone, and conglomerate. The residuum is dominantly yellowish-brown to reddiskbrown or orangish-brown with some light-gray sand. ChanneIs cut into the formerly arkosic sandstone were filled by gravel, pebbly sands, and coarse-grained sands with pebMes and cobbles mast abundant at the base of the channel fill. These larger clasts we rounded and are of variable compositions including granite, gneiss, quartz, quartzite, and cataelastic rocks. Some probabIe phyllite or schist has been completely altered to day and cannot be identified exactly. Most of the minerds in the gmnite and gneiss clasts are thoroughIy decomposed, but the original texture and composition of the rocks can still be discerned. N ear the base of the channel fill, many of the larger clasts are elliptical and their long dimensions we parallel or subparallel to the base of the trough. The basal contact is rather sharp with only minor mixing, but this contact is somewhat irregular as some of the cobbles have become partially depressed into the underlying coarse- grained sandstone. At this locality, the pebbles and cobbles are most abundant at the lowest point of the channel troughs and grade laterally into coarse-grained sandstones up the dip of the prominent trough cross-beds. The channel fill has been truncated on top, which has produced a sharp contact marked by nearly horizontal beds of coarse-grained arkosic sandstone.
Walk 110 ft (33 rn) north (uphill) dong Va. Route 730 untii the bank on the west side diminishes in height. Then turn left, climb bank, and wdk west for about 140 ft (42 rn) through the cleared out woods. After descending the slope to 8. cIeared gravelly meB, turn right and go north a short distance. TQ the right {north).are 15-20 ft (4.5-6 m) high vertical faces of a small borrow pit in the Otterdale Sandstone. During this traverse your path has been littered with abundant quarts cobbles, characteristic of cleared areas overlying conglorn erate phases of the Otterdale Sandstone. This borrow pit has been worked primarily for fill, but due to the high clay content of the residuum, it was not very satisfactory for that purpose.
These large exposures once were dominated by mars@ conglomerates, now reverting to gavels, and by dune scale trough cross- beds which are best shown on the north and east walls of the pit. At the northeast corner of the pit is a 6 in. (15 cm) wide, ver~cdclay seam cutting across the strata. This is the weathered remnant of the gouge zune of a fadt. Formerly this fault could be traced across the east side of the pit and roughly paralleled it. Displacement could not be determined, as only monotonously thick gravels are present on both sides of the fault. These cuts were once much better exposed, but their bases are mantled now by aLZuviurn and sediment deposited by slope wash. The lowest units in this exposure, totaling more than 10 ft (3 m) in thickness, are cobble-rich and appear to be . basal portions of troughs because the cobbles me dispersed in a coarse, sandy matrix and they grade into beds 1ess.rich in cobbles laterally. These are cobbles at the base of most of the obvious crossbeds as well. In general, the thick conglomerate appears to be coarser new the b&se of the exposure and becomes slightly finer upward. A few boulders more than 1 ft (30 cm) across have been found near the base of the exposures, but these are now tzsually covered by alluvium. These boulders me more angular than the average pebbles or cobbles and invariably are decomposed cornpleteIy to clay. Most appear to have been originally igneous or metamorphic rocks, but a few are reddish in color and resemble deeply weathered Triassic siltstone or shale. A few deeply weathered pebbles and mbblw are also suggestive of Triassic lithologies. The more angular boulders me set in a matrix of granules, pebbles, and cobbles of heterogeneous composition, and comse- to very coarse grained, formerly arkosic sands. Most are decomposed, but many quartz clasts remain firm and largely unweathered. Many of the pebbles and cobbles are flattened and the flat surfaces are roughly parallel to bedding. In general, the composition and weathering of the larger clasts is similar to that described at the road cuts earlier at this stop.
Above the thick conglomerate sequence is a 2.5 ft (75 cmkthick, crossbedded, coarsegrained sandstone dipping gently westward. It is overlain at a sharp contact by more conglomerate. This unit also has 8. sharp bad contact with the thick underlying conglomerates which is marked by a 2 in, (5 cml-thick layer of gray clay.
Walk about 140 ft (42 rn) toward the northwest corner of the large cleared mea facing this borrow pit and turn northward onto the narrow path which cuts through the woods. Follow this path for about 180 ft (54 m) to a large cleared mea, now growing up in scrub, which is bounded to the east or northeast by large vertical cuts about 30 ft (9 m) high. These exposures are continuous for about 300 Et (90 m) to the north and mark the edge of another very large borrow pit in the Otterdde Sandstone.
Dune-scale, trough crossbedding dominates these exposures and is well displayed at the southern end of the cut in coarsegrained, arkosic sandstones. The general lithologies, colors, and degree of weathering are similar to those shown at the other two places described at this stop; however, these exposures show a much broader’ view of the crossbedding, conglomerates, and sandstones. About 200 ft (60 m) along the wall from the south end of the cut is B wide hole cut into the face about halfway up the wall. At the very back of the hole is a poorly exposed cross section of a silicified conifer lw, probably Ara’ucarioxylon. The log is about 1 ft (3a cm) in diameter, and chips of the silicified wood usually litter the base of the hole. This log formerly was exposed at the surface of the pit face, but rock and fossil coUectors have gradually removed pieces of silicified wood over a long period of time, so now it is recessed into the wall. When this pit was being worked for fill, several such logs were exhumed and some of them were more than 20 ft (6 m) long. This log is found within 8 thick, coarse, conglomerate sequence of channel deposits, and two partid-fining upward sequences may be present. The absence of fine sediment and smaller scale bedforms as weU as the dominance of gravels and Iarge scale crossbedding suggest a braided-stream type of deposit for the Otterdale Sandstone at this Jocality.
51.9 0.0 Turn right onto Va. Route 655 (Beach Road) and proceed west on Beach Road. 52.1 0.8 Turn left onto Va. Route 603 (Beaver Bridge Road), and continue to the southeast on Beaver Bridge Read. 55.1 2.4 The center of the town of Winterpock is at the intersection of Beaver Bridge Road with Va. Route 664 (Coalbore Road). Winterpock was a coal mining center, and cod mines were active at this intersection and both north md south of it. Waste material from the mining can be seen in the saiIs and in heaps in this mea. Turn left onto Coalboro Road. 55.7 0.6 The left bank of the road is composed of waste debris from the old Cox-Clover ail1 Mine. The shafts of this mine can be seen a short distance in the woods to the west Qf the road. CARE MUST BE TAKEN IN VISITING THE SITE BECAUSE NO FENCES OR SIGNS OCCUR. AROUND THE SHAFTS. 59.6 1.2 Bear right at the fork in the road. 57.2 0.3 Turn right onto Va. Route 655 (Beach Road). 58.5- 1.3 Turn left at stop sign onto Va. Route 621 (Winterpock Road). 61.5 3.0 Turn right onto U.S. Route 360 (Hull Street Road). Coarse, boulder conglomerate was formerly exposed in low road cuts on the east side of Va. Route 621 south of this intersection, The contact between the Richmond basin and gneiss is near this intersection. 63.1 1.6 Cross over Swift Creek. Igneous and metamorphic rocks are exposed in the bed of the creek at this point. Note the deep creek valley and the absence of a floodplain. This contrasts - with the valley of Swift Creek where it was on Triassic sedirnentsry mcks earIier on this trip.
End of detailed log. Continue east 'on US. Route 360 until its intersection with Va. Route 150 (Chippenham Parkwayk go north on Chippenham Parkway until its intersection with the Powhite Parkway. From there, follow the signs to Interstate Route 195 [The Downtown Expressway), to Interstate 95 north, to Interstate 64 east to Williamsburg. Follow Interstate 64 back to Williamsburg.
END OF TRIP